Transcript for:
Understanding Atomic Structure and Mass Spectrometry

hello and welcome to this video about a level chemistry atomic structure so the chemical properties of elements depends on their at a atomic structure and in particular on the arrangement of electrons around the nucleus and so in this topic we're going to explore this idea along with others by looking at fundamental particles and what that means we'll be looking at the concepts of the mass number and isotopes will then look at electron configuration and the concepts of ionization and after that we'll finish with looking at the mass spectrometer and how we can use that to analyze different substances first up we need to take a look at fundamental particles at GCSE level you will have learned about the different models for atomic structure and over time our understanding of atoms has changed and developed as new evidence was discovered first of all we had the billiard ball model which is the idea that atoms are all like a snooker ball and they're just one solid lump and then that changed over time with the discovery of the electron to become the plum pudding model where the idea was that an atom was still a solid lump but had electrons wedged around through the atom and the metaphor is likeit's the plum pudding is like a cake and the electrons are the rest of the atom which is little tiny raisins or pieces of fruit and then that model changed with Ernest Rutherford's experiment with gold leaf and we got the nuclear model the idea that atoms have got a very small central nucleus and electrons around the outside and most of its empty space and then that model changed over time to the Bohr model of the atom and that consisted of the electrons arranged in energy levels so the nucleus of the atom Taine's the protons and neutrons and these are held together by what's called the strong nuclear force which is a as the name suggests the strong nuclear force is a very very strong force far stronger than the electrostatic attraction which is what we get that holds these electrons in place around the atom this is the electrostatics and the nuclear force is much stronger than the electrostatic force that holds these electrons into place around the atom and so these protons and neutrons are held together by the nuclear force and they are referred to as nucleons so the things in the nucleus are referred to as nucleons when we look at protons and neutrons and electrons and they are absolutely tiny and so when we're thinking about how big they are we don't look at their actual mass because the actual mass of a proton is something like one point six seven times 10 to the minus 27 kilograms and their charge is even smaller in terms of the coulombs of charge so we compare them and we say a proton and neutron have got the same relative mass and we give that a mass of 1 and when we compare it to electrons electrons are almost zero but not quite it is 1 over 1840 so we need 1840 electrons to equal the mass of one proton that's what that means in terms of charged protons they've got a mass of a charge of +1 and electrons have got a charge of minus 1 which is why in an atom the number of protons and the number of electrons are the same because we end up with no charge overall because the protons and electrons are equal in an atom and last of all neutrons are neutral neutrons have zero charge if we move on there to have a look at how many fundamental particles there are in atoms or ions do on what we're looking at we need to have a recap of how we can use the periodic table to help us do this so remember all elements have got their own symbol and then they've got some numbers everywhere elements got two different numbers the numbers are called the atomic number and that is given the symbol Z and we've got the mass number which is given the symbol a now the atomic number is the number of protons in an atom and the mass number is the number of nucleons that's means the number of things in the nucleus and so it is a number of protons plus the number of neutrons so it's the sum of those two values so if we have a look at fluorine down here in terms of protons fluorine will have nine now we said on the previous page that for atoms the number of protons and the number of electrons is the same so that nine will be for both of these zinc will be exactly the same nine protons fluorine thirty protons for the zinc and thirty electrons as well where we'll get some differences over here for sodium because that one plus tells us that sodium is an ion and more specifically it is an ion because it has lost one electron and so what that means is it's got 11 protons because that's its atomic number but it will only have ten electrons this time because it has lost one if we have a look at neutrons then neutrons is the total number of protons plus the number of neutrons so if we've got nine protons in fluorine here and we've got an atomic mass of 19 a mass number of 19 then the number of neutrons can be found by doing 19 take away 9 which is 10 and we can do 65 take away 30 which is 35 neutrons and here the number of neutrons is 23 take away 1211 which is 12 on there so the neutrons is the atomic mass number take away the atomic number over there the big number so quite a small number on there we're going to have a look now at some more complicated particles called isotopes now isotopes are particles that have got the same number of protons but different numbers of neutrons and so I've got some examples here for us to work out so the protons in these substances nice and easy six for all of them because the atomic number is six every single time the electrons is also six for every single one of these and the neutrons this time is going to be different twelve take away six is going to give us six neutrons for this one 13 take away six it's going to give us seven neutrons on here and 14 take away six is going to give us eight if we have a look at bromine down here and bromine has got 35 protons in both cases it's got thirty-five electrons in both cases and this one is going to have 44 neutrons and this one is going to have 46 neutrons on here now a really important thing about isotopes is their reactivity and actually reactivity is governed by the one particle which isn't mentioned in the definition of what isotopes are which is electrons and the fact that these three isotopes of carbon have all got the same number of electrons the they will react in the same way chemically irrespective over the fact that they're slightly different masses on there and the same would be true for the bromine those two bromine atoms would react in the same way because they have both got 35 electrons we're going to move on there to have a look at electron configuration which is a fancy way of saying how are the electrons organized around the nucleus of an atom now we know at GCSE that these are organized in shells and you might have also called them energy levels too and we'll find out a little bit later why we bother calling them energy levels instead of shells now for carbon carbon has got six electrons and it's got two electrons in first shell and it's got four in the second shell so simple electron configuration sort of GCSE level would have had was listing them as two comma four now lithium which has got three electrons would put two electrons in the first energy level or shell one in the second one and the electron configuration would be this lithium plus which once again is an ion it's positive still so that means it's lost an electron it would have only two electrons and so its electron configuration would be two with a plus like that now this idea has evolved into a more sophisticated model mostly because we discovered that electrons in a particular energy level are actually not all the same now at GCSE level you're keeping the model simple so this is what we've considered so far but what we're going to move on to you now is looking at the slightly more complicated model so first up if we consider the electron shell model again we know from GCSE level that this shell can hold two electrons and the second shell can hold eight electrons now what you were not taught GCSE level is actually the third shell can hold eighteen electrons and not actually eight which is what you were taught but what we were careful to do is only consider the atoms up to number twenty and so this rule this extra complication didn't actually come up it wasn't an issue on there now the electron shells that we've just been looking at the first the second and the third they will all have different amounts of energy because of how far they are away from the nucleus the first electron shell has got less energy than the second and that has got less energy than the third and that is why we can call them energy levels instead of shells because the first shell has got less energy and the second has got a higher energy level and the third is a higher energy level still this gets even more complicated because electrons in different shells have got different amounts of energy and so we can take our energy level diagram and actually add an extra level of detail to it instead of just looking energy levels we can look at sub levels sometimes also called sub shells now first ones nice and easy the first sub level is the same as the first energy level but it immediately gets complicated after that first level because these levels are divided into the sub levels which have got different names and their names are s P D and F they don't all have all four of those different names but they do starting from the second energy level start to have some variety so in the second energy level we've got the S sub level and the P sub level and they've got different amounts of energy so the electrons in the second energy level arms all the same the electrons in the third energy level arms it all the same either you've actually got three sub levels on here we've got the S we've got the P and we've got the D sub levels on there just to go back to the number of electrons that an energy level can hold there's actually a rule that you can remember where the number of electrons is equal to 2n squared where n is the number for the energy level and so what that means is that if we are in the second energy level we've got to lots of N squared so 2 times 2 is 4 then size by 2 again gives us our 8 that we have in the second energy level and for the third energy level we've got 3 squared because it's n squared which makes 9 we're then doubling it which makes us our 18 electrons that we can have in each energy level we're going to look in a bit more detail now at what these sub levels are actually like and before we can do that we need to recognize in fact that electrons as we know it's at GCSE we don't quite consider them in the same way we don't consider them to be a particle anymore but a cloud of negative charge and an electron fills a particular volume of space called an atomic orbital and different atomic orbitals have got different energy levels and each atomic orbital has got a number that tells us what energy level it corresponds to so that might be a 1 or a 2 or a 3 but the atomic orbitals have all got different shapes as well which in turn have got slightly different energy levels and these are called sub levels that I've mentioned before and we refer to these as s or P or D or F and these orbitals have all got different shapes which I'm going to have a go at drawing now so first up the S orbital is actually just a spherical shape so that's nice and easy to remember s for spherical the next ones are P orbitals now P orbitals there's actually three different types of p orbital and they each follow one of the XY & z planes on here so that's a px orbital and then we've got a PZ orbital that goes on the z-axis up there and then last of all we've got a P Y orbital on there so those three different P orbitals that we've got and then the last type of orbital that you need to be aware of is the deorbit somehow they're really complicated to draw so I'm not going to draw them whilst doing this video but except to do one which is my favorite one which is where we've got one part of the orbital goes up the z-axis and then another part sort of circles around like a hula hoop of favor there but just we need to know that there are five different D orbitals on there now let's take a look at these orbitals represented in our energy level diagrams as I said before the first energy level consists of a single s orbital and that's what we're showing on here and the second one is split into 2 s and of 2p and then the third one is split into 3s 3p and up here the 3d on here now we've got some rules that we need to recognize so first up each atomic orbital can only hold a maximum of two electrons and the S orbital since there is only one orbital in the s sublevel and each one can only hold two electrons then the number of electrons in two so in an S orbital in the s sublevel is - in the P sub level because there are three different orbitals in the P sub level as we can see down here that means the P sub level can hold a total of six electrons and then the D sub level because there are five different orbitals in the D sub level that means that the D sub shell can hold a total of 10 electrons on there and then if we quickly know that in the third energy level there is one p subshell one s subshell on one d subshell then it's easy to see now why the third energy level can hold a total of 18 electrons in those different nine orbitals on here one last other thing to note is actually the slightly weird thing which is that the 4s has got less energy than the 3d because each one of these is ranked in terms of increasing energy as we go up here they have more energy more energy more energy and the 4s has got less energy than the 3d although that does change when ions are formed just a little bit more data about electrons electrons are said to have a property of spin now it's really important to note that the electrons aren't actually spinning but it is to do with their revolutions that they make around the sensor nucleus now to do with this property of spin we've got two electrons in the same orbital and they have got opposite spins and what we do to represent that is we use arrows and we use one arrow up we call that spin up and we have another arrow pointing down and we call that spin down now let's have a look at how we put electrons into their places around the nucleus of an atom so there's three rules here mostly there's two rules but I've called it three so the first rule is that you must always fill the lower energy levels first that's because the lower energy levels are more stable and that's where they will go the electrons the second rule is that when you're putting electrons into orbitals you fill them singly before you pair them up so that means you put them in one at a time and then there's a rule 3 which is just a reminder rule really which is that you put two electrons into an orbital and then it is full now let's have a look at an example on here if we consider first of all carbon which has got six electrons to put in we've got the one s subshell we put two electrons into there and then it is full we've got four more to put in so we put two into the two s sublevel and then that is full and then last of all we've got two more electrons and we put them into our 2p sub-shell on there we don't need to fill that up and if we have a look at oxygen which has got eight electrons it's the same thing here with the 1s the same thing here with the 2s and then when we get to the 2p because because we've only used four electrons down here we need to put another four into the 2s sub level on here and so we've put 3 in and that's us single-occupancy and so we don't have a choice now we have to put the fourth electron into there let me get this double occupancy which does affect the quality of oxygen that we'll come to later we can write the electron structure or configuration in a similar way but more complicated so we were doing a GCSE level so lithium has got three electrons and so we need to put those into their sub levels so the first two electrons will go into the 1s sub level and the second sub level that is now needed to have one electron in it and that is now full what you've got is you've got the powers in a way because they're always up in the air numbers they're not down on the line that is equal to the total number of electrons that we've got in our particular atom so three for lithium on here and if we have a look at sodium which has got 11 electrons in total that will be 1s2 then that's full to s2 then that's full to p6 and that's so far if we're keeping track is 2 + 2 + 6 which makes 10 which means we need one more electron and 3's one gives us our 11 electrons in total for carbon we have got 6 electrons so we write 1s2 2s2 2p4 and that's then done we don't need to do all this up here this was just me keeping track of the total number of electrons for you don't write that in the electron configuration for argon argon is actually got 18 electrons so that's a little bit lengthier and that's the first 10 electrons done on that and then the next 2 and then the next six and that's done on there that's a total of 18 electrons upon here iron has got even more electrons it's got 26 and so the first 18 will be exactly the same as argon so now extra thought needed on there and then we've got another 8 electrons to put in and so we need to get to that rule where you fill the 4s electrons before you fill the 3d so we've got 4s 2 3 D on there and that's our total number of electrons now even though we fill all the 4s before the 3d what we should really do though is we should not have the 4s first we should have the 3d six first and the forest so because as soon as they become occupied the 4s actually gains a little bits of energy which causes an issue that we're going to come across in just a moment last of all to do with these electron structures electronic structures we can indicate something about our elements that we have been looking at and we can refer to them by the identity of their outer electrons so lithium and sodium are referred to as s block elements because their outer electron is in an S sub level and carbon and argon are referred to as P block elements for the same reason that outer electrons are in a P sublevel ayan is referred to as a D block element because it's after electrons are in a D sublevel it's got an incomplete D sub level which is one of the characteristics of the transition elements and you can navigate your way around that by looking at the periodic table because the s block is everything in Group one and group two the P block is group three to group eight and then the D block is the transition elements on there or the transition metals they're sometimes referred to now as I've already said about transition metals there are two rules about the transition metals you always put the 4s electrons in before you put the three D in although you do usually write the three D first as you're listing them and then you remove the electrons from the 4s before the three D so that's relevant if you're working at the electronic structure of some particular transition metal ions there are two exceptions to the filling rules that you need to be aware and those exceptions are for chromium and for copper so you would expect chromium to have the electron structure 3d for 4s2 and you'd expect copper to be 3d 9 for s2 but actually that's not right because chromium and copper would get more stability if they do something slightly different and what they have is they have 3d five for s1 for chromium and 3d 10 for s1 for copper and that precisely for the energy sub level and that heartful d sub level gives the elements extra stability that they wouldn't have if it was almost full or almost half full so these are actually more stable and so this is the organization that we get for chromium and for copper in terms of their electrons and those are two exceptions that you need to know of now let's have a look at ionization energy the ionization energy obviously it's the amount of energy because that's in its name but it's the energy required to remove one mole of electrons from a mole of gaseous atoms now what does that actually mean well what that means is the electron that gets removed is no longer a part of that atom anymore so if we just consider something simple and small let's do lithium on there it's got three electrons if we're going to ionize lithium we have to give lithium more energy to get this electron to jump not just up a few sub levels or a few energy levels but we actually have to get it to jump so far that it has left all the energy levels behind so we've given it enough energy to make that leap from what's called the ground state where the element it has its electrons normally to those electrons on part of that atom anymore and this energy level infinity that's kind of considered to be the outside of the atom so when you're ionizing something you're giving the electron enough energy to leap from where it normally is in the ground state passed all the other energy levels pass the outer energy level and it's gone and so that's the ionization energy of a particular substance and what you can see from this diagram is if some things outer electrons were in a D sub level the amount of energy needed to make that electron leave the atom actually wouldn't be as much energy as if it was down here in the 2s because that amount of energy that energy changed given the symbol Delta e is obviously different depending on if you're in a higher energy like this one or a lower energy than that one and so I inhalation energy there is from atom to atom and these numbers that we have they're all measured with the units in kilojoules per mole of that substance kilojoules per mole in terms of ionization energy I was just looking at taking one electron away from a particular element but you don't always have to limit it to one electron you might take away more than one electron now the first of that turn you take away is called the first ionization energy and the equation that you have to write to represent the first ionization energy always has the elements on the left-hand side the element has to be in the gaseous state and then it turns into an ion which is also in the gaseous state and we lose an electron that's the first ionization the second ionization energy is very logically you've taken one of that runaway and so you've got your gaseous ion we're going to take another electron away from it as well to leave us with a gaseous ion that has lost two electrons and so is therefore a two plus ion and we get our final electron on there just the one electron on here and last of all for my examples the third ionization energy we take an ion that's already lost two electrons so it's a two plus ion it loses another electron and so it becomes a three plus ion this time losing another electron on here in terms of comparing these ionization energies to each other the first ionization energy is always the smallest number and that's because we're taking electron away from a neutral atom then the second ionization energy is larger than the first and that's because the electron is being taken away from something that is a positive ion and so what that means is because it's a positive ion we've got greater electrostatic forces of attraction because we've got something that is positive and we're trying to take that negative electron away from it and there's really strong electrostatic forces on here and it follows that the third ionization energy is going to be larger still because we're taking an electron away from something that is two plus so it's got an even stronger positive charge an even greater attraction for electrons now what can I annoys a ssin energy be used for so the first thing is it provides us with evidence or the electron shell model for atomic theory and so if we consider sodium which has got 11 electrons in all of its energy levels we can investigate what the ionization energy is for sodium with each successive electron that we remove so the first electron that we remove is really easy to remove it's on its ionization energy is very very low the second one is harder as I've said you're taking an electron away from something that is positively charged now the successive next eight are all quite similar to remove gets a little bit higher but not massively each time and now it's easy to see the pattern on here that we've got one electron that's very easy to remove and that's in the third energy level and then we've got eight electrons along here which are reasonably easy to remove but they get harder as you remove more and more of them and that's because we're taking electrons from only the second energy level and then the first energy level on here these are the ones that are hardest to remove because they are very very close to the nucleus and so they are experiencing a great deal of attraction from the positive nucleus whereas as we go down here this third energy level these are really really shielded electrons from the nucleus as pull we've got a lot of different kind of layers in the way blocking the positive attraction of the nucleus to get to those outer electrons in the third energy level now even cleverer than this is if we consider just the electrons in the second energy level and this time take the log to the base 10 of those ionization energies and we plot them successively what we find this time is that we get six electrons which follow a pretty straightforward pattern on there of ionization energy or log of ionization energy increasing as you take one electron than another but then we get a leap for the final two so even those eight electrons on here aren't equivalent and what that gave us our evidence for is that these six electrons that are pretty close together in their pattern they're 2p electrons whereas these ones on here are two s and because the 2's are lower energy what that means is they are therefore harder to remove because we have to give them more energy to get them to leave the atom and of course they're also closer to the nucleus as well and so that means that they experience a greater Electress electrostatic attraction from the nucleus as pop we can also use ionization energies to work out what group of the periodic table a particular element might be in what we have to do to do that is we have to look at successive ionization energies and so if we have a look here we've got our first element where the first ionization energy is 1,000 the second ionization energy is 1,500 then two thousand and eight thousand then nine thousand and the skill that we have to employ here is we're looking for a big leap a big gap and so for me the big gap here is between the third and the fourth electron removal and so that means that the first three electrons are pretty similar to remove as you go along they get a bit harder but we're expecting them to get more difficult to remove and so what this tells us is that this has got three outer energy level electrons which therefore means it is in group 3 of the periodic table and if we have a look at this next example here for a different element we can see we've got 500 we've got 4500 well that's a big leap already because that's got 9 times bigger so it's a leap of 4000 it's got 9 times bigger if we have a track on here just looking here this is actually a larger increase of 5,000 but 9,500 to 14,000 is actually only in about 50% where is this it got 9 times bigger so that was a much bigger increase on here so that means that this one electron is present in the outer energy level and so this is therefore in Group one the final thing about ionization energy is that you need to know for a level chemistry is you need to know about patterns across period three and down group two now across period three if we consider ionization energies the pattern is one of two three three that's quite helpful to remember and so first up if we just start there's no numbers on this graph this is just showing the general trend it is an increase of two in there or there's two in an increasing pattern then there is a dip and we've got three in an increase in pattern and then there's another dip and we've got our last three elements of Period three in an increasing pattern so overall ionization energy increases across a period and the reason for those increases there's three reasons or two reasons and then end result and so first up is that we get an increased nuclear charge for each element as you grow across the period and secondly the atomic radius decreases each time we move along one element in the periodic table and then thirdly it's kind of a reason is that the electrons are in the same energy level and so that means that the shielding force that I mentioned before is the exact same for each of these elements as we go across the period and so overall there is therefore a stronger attraction between the nucleus and the outer energy level electrons so this is greater attraction between the nucleus and the outer electrons because not just any hot electrons are being removed it's the outer electrons there are however two dips in our pattern the first dip is easier to explain and when you're asked to explain the cause of a dip I think it's really useful to work out what the electron arrangement is for the different elements that you're comparing so we're in period three so that means the second element in period three has got its outer electron in the 3s two sub shell whereas four here because we've got the third element in group 3 it will be 3 s 2 3 P 1 which means that the outer electron is being removed from AP sub shell for this element on the right whereas the outer electron is being removed from the S electron sub shell for the elements on the left and we know that P is a higher energy level or higher energy I should say than an S and so therefore it is easier to remove over here this dip in this region is a little bit harder to explain but if we have a look again at the electrons is 3 s 2 3 3 3 and this one is 3 s 2 3 P 4 and so what we're comparing is P 3 2 P 4 now if you remember that rule about the electrons occupying singly before EE what that means is in the orbitals this element on here has got three single occupancy orbitals whereas down here this 3 P 4 we've obviously only got room for 3 electrons to occupy singly so the fourth one has to pair up and because of this thing called double occupancy these electrons are easier to remove and that's the reason for that second dip as we move across period 3 group 2 is actually a little bit easier it just looks a little bit complicated because of the first element that we have and then the second and then the third that's a nice regular trend as atomic number increases and then it stretches out and it looks a little bit weird that it's stretching like this but actually the only reason for that is that we've moved into the d-block and so the number of electrons in a particular energy level increases from 2 to 818 so the pattern isn't quite linear on there but the general rule certainly is and that is that ionization energy decreases down a group and the reason for that decrease is really similar to the justification across period 3 the atomic radius though is increasing and as a result of that increase in atomic radius there is weaker attraction between the nucleus and the outer electrons but as well as that shielding is also increasing so that means that the electron shells are blocking the nucleus attraction for the outer electrons and both of those factors end up with a weaker electrostatic attraction between the nucleus and the outer electron which is potentially being removed in the ionization we're going to move on now and have a look at the mass spectrometer which is almost a separate section but I've put it all in one video so a mass spectrometer is a machine and it's a machine for measuring because it's got the wave meter or part of the word meter at the end and that's a clue that it's measuring something and what it's measuring is the mass of a particular atom or a molecule and I'm not spectrometer is very very very accurate and so it gives us a very precise relative atomic mass and so the first thing that it can measure is it can measure the relative atomic mass and it can measure the relative molecular mass for a particular substance which is really useful the second thing it can do is it can identify a particular element from its relative atomic mass value because most elements have got a unique relative atomic mass and so if you know something's relative atomic mass you can be pretty confident about it's identity although not always just want to take this opportunity to give you the definition for relative atomic mass and relative molecular mass they're really really and what you do to calculate the relative atomic mass is you take the average mass of one particular atom and you compare it to the massive 1/12 of the carbon-12 atoms so it's a twelfth of the mass of a carbon-12 atom and then the relative molecular mass is very very similar only instead of it being the average mass of an atom it's the average mass of one molecule but we still compare it to the exact same thing we compare it to a twelfth of the mass of a carbon-12 atom and we use carbon-12 because it is pretty high isotopic purity so it's about 99% of all carbon is carbon-12 and so we can be quite confident in our comparison whereas other isotopes have got a far more mixed proportions on there so it's really important to know that we're comparing it to carbon-12 atom and because carbon-12 has got a mass of 12 what we're doing is we're comparing it to a twelfth of the mass of one of those atoms because that allows us to get our value of one if we take a twelfth of the mass of something that's got a mass of twelve now what happens in the mass spectrometer it depends on the mass spectrometer that you're looking at what you need to know for your a level course is a time-of-flight mass spectrometer which has got four main steps and the first step is ionization and unsurprisingly in that step we make ions and we need to know that we make positive ions in that step and I've got a pretty crude sketch of the mass spectrometer but the samples injected here and ionization happens in this stage on here so here's our sample and here's a sample and then it gets ionized so it becomes civilly charged then the ions that are produced all accelerate across the gap we've got a negatively charged plate over here and obviously positive things and negative things are attracted towards each other and this plate over here is fixed so it can't move so these ions will drift across the gap on here so that's what happens in here we get acceleration because the ions gain kinetic energy and they all gain the same kinetic energy so all ions have the same kinetic energy and what that means because they've all got the same kinetic energy is that the ions that are lighter will travel faster because if you push two things and ones light and one's heavy the lighter one will move faster and so stage 3 which is where the ions move along the tube the lighter ones are going to move along the Trib tube with a greater velocity when they get to the negatively charged plate here there's actually a gap in that plate that allows the positive ions to pass through and that kind of focuses the beams that drift along here and then the final step in the mass spectrometer is the ions hit a detector on here and as I said the lightest ones arrive first and the heavier arrive last and then the detector sends a reading to a PC which gives you your display for your mass spectrum and what I should say to finish off this little side tube here this is to a vacuum pump on here and what that does is that sucks all of the air out of the tube and that prevents our ions from colliding with air and being deflected off the path and never hitting the detector just once have a look at in a bit more detail about what happens in ionization there are two types of ionization and you need to be careful when you're answering exam questions paying attention to which type of ionization they are asking you about there is the ionization caused by an electron gun which is what was on the old course and so that you'll find more pass paid the questions about the electron gun method and what happens in the electron gun if you remember the sample has been injected over here and we've got no charge no charge and what happens here is what we've got a filament of wire that is very very hot and that is a source of electrons this hot filament and what happens is the electrons get fired across this gap over here so what is positively charged plate and the sample that's moving through the flight tube gets hit by those electrons and more electrons get knocked off so we've got our gaseous sample because the sample that passes through the tube has got to be a gas and we hit it with an electron and that knocks another electron off so we end up with a gaseous ion and we end up with two electrons because we had one that was from the gun and another one that has been knocked off alternatively you can write this equation slightly more simply by removing the electron from the electron gun and just only writing down in the equation the electron that was knocked off from the sample then the second technique is new to the course for a few years ago so you've got fewer questions about this electron spray technique and this also happens in the early part of the flight tube and what you've got here is you've got a needle so the sample gets injected but what happens is it gets dissolved in a volatile solvent and then it's forced through a needle and as its forced through a needle a high voltage is applied and what that causes to happen is the sample gains a proton and so what we have here is we've got our sample which I'm just going to write as X again and it gains a proton and so what happens is we end up with a slightly weird-looking thing which is our sample with a proton joined to it an H+ joined to it so it's slightly heavier than before which is worth noting in a minute and then last of all the solvent evaporates to form gaseous positive ions to form X h+ as a gas and so the ions have drifted through the tube and they come towards the detector over here now remember all ions get the same kinetic energy which means the lights or ions get detected first they've got a shorter time of flight that's where the phrase time-of-flight mass spectrometer comes in they detected first they have got a shorter time of flight and so the ions are drifting towards the detector they hit the detector there's a positive ion hitting the detector and so what happens is the positive ions pick up electrons from the detector and that causes a currents of flow now if we've got a really abundant particular ion a greater number of electrons will move off the detector and move towards the ions and so we'll get a greater current and so our conclusion on here is if we get a larger current we've got a particular ion or a substance that has got a greater abundance that means there's more of it in our sample so what does a mass spectrum actually look like so we've got two spectra on this slide on here the first one is sure one particular quality of mass spectra which is where you have isotopes you get a peak for each isotope that is present and so these are isotopes of chlorine and so we have some chlorine which has got a mass of 35 and we've got other chlorine that's got a mass of 37 and that's coming up on this x-axis as a MZ ratio and that's referred to as a mass to charge ratio and that mass to charge ratio is really quite simply calculated because normally if one electron is removed or one proton is gained that means that the charge is going to be one plus and so if the charge is one plus that means that Z the charge is one and so that means that MZ is effectively the M mass if the charge is one and so what that means to go back over here is that we've got chlorine with a mass of 37 and chlorine with a mass of 31 now I've tried to draw these two scale because what we can learn on here is we can track across at the abundance and we can find out that 75 percent of all of our chlorine 35 and then 25 percent of all of our chlorine is chlorine 37 the slightly heavier isotope of chlorine abundances might be a percentage or they might not it might just be a comparison because 75 to 25 could just be 3 to 1 as a proportion on there doesn't have to be a percentage the second useful thing about mass spectra is shown on this second spectrum down at the bottom and that involves the peak that is the furthest to the right in a mass spectrum which is the heaviest peak the heaviest peak the one with the greatest MZ ratio is always the molecular ion peak and so what that means is you've got the whole molecule that has either had one electron knocked off it or it has gained one proton and so this molecular ion peak is absolutely vital because it tells us what the M R is for a particular substance so for here for my made-up sample my made-up spectrum we've got an M R of 112 from this substance and that can help us to identify what that substance is another little quality of mass spectra is we have Peaks not just the molecular ion peak but we also have slightly lower NZ values and this comes from something called fragmentation and these are Peaks for particular fragments of the molecule and that's because it's particularly common in the electron gun ionization when an electron gets removed from a covalent bond that's holding two atoms together we've only got one electron holding the atoms together and that isn't really enough attractive force to hold those two atoms together so they fall apart in two pieces and one of those pieces will be positively charged and it's that that gets detected in the mass spectrometer just to look briefly it's slightly more tricky mass spectra the first one isn't really particularly tricky it's just a notable difference from the previous mass spectrum on the previous page and this one has been generated using the electron spray mass spectrometer and so when you are in eyes using that remember that what you detect is the gaseous sample that has gained a proton so you're detecting something with an M ZL u that is one higher than whatever the M R is so this 113 over here that is for the substance with an extra proton a hydrogen ion and this hydrogen ion has got a mass of 1 so that means that the M R of this substance is actually 113 take away one not complicated maths but you might forget to do it so you need to take away one from the NZD when you're working out what the relative molecular mass is for a sample if it's been ionized using electron spray that's what I was saying when I about paying attention to what ionization technique they mentioned in the question and then last of all a tricky mass spectrum could be for when we have a molecule of chlorine now this is tricky because there's two atoms in CLC obviously but they could both could have different maths masses so they both could be different they might not be different but it could be and so what we could have is we could have if we consider say chlorine atom a and chlorine atom B chlorine a could have a relative atomic mass of 35 and so could chlorine be chlorine a could have a mass of 35 chlorine B could be 37 chlorine a that could have been 37 and chlorine B could have been 35 and then the last combination as possible is 37 for both of them and so what that means is that when we combine those two together that has got a total mass of 70 that's 72 that's 72 as well and that's 74 so the mass spectrum for chlorine will have three peaks one at 7c one at 72 and one at 74 and they won't all necessarily be the same height in fact that definitely won't all be the same height in this instance on here and the reason for that is kind of statistics and in maths that three-quarters of all of the chlorine is chlorine 35 so that means there is a 3 out of 4 chance that this atom over here will be chlorine 35 and a 3 out of 4 chance that this one will be as well so that means that there is a 9 out of 16 chance that they will both in Corinne 35 the probability that the first chlorine atom is chlorine 35 is 3 out of 4 if the second one is going to be the heavier chlorine 37 isotope that is not so likely and so there is a 3 out of 16 chance of this combination the third combination is the exact same odds just the other way around and so that's a three out of sixteen chance on there and then the last combination it's actually only a one in 16 chance that we're going to get that so if we were to draw these Peaks the chlorine 37 37 so the one with an MZ of 74 would not be very high at all its abundance would be 1 or 1 out of 16 but I'm just going to call it 1 by taking the first digit in my fraction the peak at chlorine 72 because we've got to the combined together we've got a 6 out of 16 chance that chlorine will be chlorine 72 CL 2 with 17 and so that means that this abundance would be a size 6 and then last of all the greatest probability because it's 9 out of 16 well that tells us that it needs to go all the way up to here to what would be 9 for the abundance on here so we would have three peaks and the chlorine 70 would be the tallest peak of all followed by chlorine 72 followed by chlorine 74 they would be in terms of their abundance it would be a 9 to 6 to 1 ratio let's have a look now at mass spectrum calculations there's two equations that you need to be able to use and you'll probably get given them in the exam but it's worth getting familiar with them in case you don't get given them and so the first one is the the equation that you learned GCSE level where we're looking at speed equals the distance divided by time now we use the symbol V because what we're really talking about is we're talking about the velocity that these ions are moving through the mass spectrometer and that is in meters per second T is the time of flight and that is in seconds and D is the distance that's really the length of the time-of-flight mass spectrometer and that is in meters ke is the kinetic energy and that's in joules remember that is the same for all of the ions that can be really important v this is once again the velocity of the ion in meters per second but then we're going to square it and then last of all this n this is the mass of one ion and slightly confusingly it's the mass of one ion in kilograms so what that means is if we know the relative atomic mass what that tells us is the mass in grams of one mole and so what we then need to do is we need to divide that by a thousand because that will give us the mass in kg of one mole and then we need to divide by Avogadro's number six point naught to two times 10 to the 23 because that will then give us the mass in kilograms of one atom because in one mole there are six point naught to 2 times 10 to the 23 atoms and so we divide by that number to find the mass in kilograms of one atom it's quite complicated to keep saying that number again again again so Avogadro's number has got the letter L capital L if we wanted to use that in calculations just by the by because we're talking about ions even though it is an ion that means it's lost an electron because the mass of an electron is practically zero that means that we can assume that the mass of an atom is the same as the mass of the ion that it would form just a quick note about rearranging these equations so speed equals distance divided by time so that means we've got distance divided by time and we've got the V on there and so that is a formula triangle that you're familiar with from GCSE level and then kinetic energy is half MV squared some people like to just remember the rearranged formula because they don't like rearranging things and so that means the velocity is two times by the kinetic energy divided by the mass of one ion in killer now remember whenever you do one of these calculations particularly if you're working out time-of-flight to do what's called a reasonableness check and what that means is does my time seem feasible because if I've done a calculation and I've been given the time of flight for one substance and then I'm calculating the time of flight for something else that is heavier then I need to be aware that the time of flight for an ion that is heavier should be greater than the time of flight for something that is lighter might not be very very different but it should be heavier then the numbers are often really quite small it doesn't take very long for the ions to go through this tube so just be aware that they were the answers likely to be in standard form and very very small the last thing about calculations in the mass spectrometer that I wanted to show you is a shortcut and that shortcut can be used by making several kind of connections between these two equations and between what happens for one isotope and it being the same when we're considering another isotope the first thing that we need to do is we need to consider the kinetic energy of each of these isotopes the kinetic energy is half MV squared for each of them I'm just using little subscripts one and two to show the masses of the two different isotopes and masses and the velocities of the two different isotopes on there so kinetic energy is MV squared for both of them now since it is the same for both we can merge those together and no longer have the kinetic energy in the middle 1/2 MV squared for the first isotope is going to be half MV squared for the second isotope then I'm going to make it slightly more complicated before making a lot easier so if we substitute in that calculation for how you work out the mass of one atom or one ice ion I should say by taking the relative atomic mass and dividing it by a thousand and then dividing it by Avogadro's number what we're going to end up with is we're going to end with half times by the relative atomic mass of the first isotope divided by a thousand times by Avogadro's number multiplied by the velocity of that isotope squared is equal to 1/2 times by the relative atomic mass of the second isotope divided by a thousand times by Avogadro's number and then the velocity of that ion squared and then we need to substitute in one last time for V speed is distance divided by the time so when we substitute that in we've got half times by the relative atomic mass of the first isotope divided by a thousand L multiplied by the distance and then the time of flight is on the bottom over there that t1 is the time of fly for iron number one and we do the exact same thing for the second isotope and we're left with an equation that looks really complicated but it's about to get really simple because whenever you've got something on one side of an equation and you've got it on the other that is the same on both sides we can get rid of it so there is no point multiplying this number by 1/2 on both sides so we can get rid of the halves because that's present on both sides we can get rid of the thousand on both sides because that's the same on both times and then we can get rid of avocados number because that is a constant one of the fundamental constants in chemistry we can get rid of that from both sides we can also get rid of the D from both sides because both of them both of the ions will be passing through the same tube and so they'll be traveling the same distance and so D will be a constant value as well so once all those numbers have been taken away we've got the relative atomic mass of the first ion divided by its time of flight squared will be equal to the relative atomic mass of the second ion divided by its time of flight squared and so that little box there is a really nice kind of shortcut memory aid whatever you want on it a way to help you work fast particularly if you're calculating the time of flight for a particular isotope and that can rearrange by the way if you like it in a rearranged form because you don't want to remember something that is hard to rearrange then the time of flight of the first isotope is the square root of the atomic number of the first isotope times by the time of flight of the second isotope squared divided by the atomic number of the second isotope on there so that's another form for that same equation let's finish this video by considering a couple of last few things about isotopes so the first thing to consider is something that you will have known for some time which is that chlorine has a relative atomic mass that is not a whole number and this is an average relative atomic mass and what that means is if we had 100 chlorine atoms and we put them all in a box and we found out how heavy they all were together the total mass of those 100 atoms and then we divided it by 100 to get our average the number that we would have is 35 point five but you would not find a single chlorine atom that had a mass of 30 5.5 what you would find as I mentioned on a previous page is that you'd find chlorine that had a mass of 35 and chlorine that had a mass of 37 17 protons for each of them by the way and the abundance of those I showed you on the mass spectrometer page is 75 percent of this one and 25 percent of this one and so what that means is how do we calculate our average well the relative atomic mass is found by taking the abundance of one isotope multiplied by the relative atomic mass then we add it to the abundance of the second isotope times by that relative atomic mass and then we divide it all by the total abundance and if there's more than two isotopes we just kind of keep going on and on on for however many that there might be three four can't imagine there'd be more than three and an exam question that would be a bit mean and so what that means is that we can do a sum where the average mass the chlorine is 75 x 35 added to 25 times by 37 and then all divided by 100 over there and that gets us our 35.5 as our answer now if the abundances aren't given as percentages we do the same thing so 75 to 25 as I said before is a three-to-one ratio of three to one proportion and so the total abundance doesn't always have to be 100% like it is here the total abundance could be four as it is in this instance and so the sum that we do here would be the same principle we do three x 35 and we'd add to that one times five 37 and instead of dividing by 100 we divide by four and we'd get the same answer of 35 point five and good to note by the way three significant figures or the number of significant figures that you've been given in the question but if in doubt three significant figures is a good rule of thumb and the last aspect of isotopes and their masses is what about if they don't ask you what the average mass is but what if they give you the average mass and so you can see here that we've got the mass of iron is 55 but there's another isotope that's got a mass of 56 and the relative atomic mass is fifty five point eight so that is the average on here now there's two different ways to do this I'm going to show you the one that I think is the easier when first I think it's faster as well and that seems a bit weird but I'm going to make a number line and I'm gonna put 55 at one end and I'm going to put 56 at the other end and I'm going to divide the rest into one two three four five six seven eight nine chunks and splits into ten by dividing it nine times and so what we need to do is we need to mark on fifty six fifty five point nine fifty five point eight on there and then how we use this number line is we take steps from one end towards where the average is so this is the average on here and we've split it into ten chunks and so we've moved 8 out of 10 chunks eight out of ten steps towards fifty-six and what that means is eight out of ten atoms of iron are 56 so this is 80% Fe 56 and if we start at the other end if we start here and we go one two we take two of the ten steps towards 55 and that tells us that it's 20% I am 55 alternatively if we have a look at the chlorine example from before we've got chlorine 35 and we've got chlorine 37 and we don't know what the abundance is but we know that the average is thirty five point five we could split it this time we don't always have to split it into 0.1 we split it into whatever makes sense and so that's 35 then thirty five point five that's 36 and that's 36 point five so I've split this into four chunks this time and the average is here 35 point five so we've got quarters and we move one quarter of the way towards chlorine 37 and that tells us that one quarter of all of the chlorine is chlorine 37 if we started at the other end we move one two three quarters of the way towards chlorine 35 and that means it is three quarters chlorine 35 last of all this question you could solve this using a number line but I think it's probably a bit easier not to on this occasion and so this question would say magnesium exists as three isotopes magnesium 24 25 and 26 the average relative atomic mass is 24.3 as shown there and they'd give you one fact they'd say right magnesium-26 is 10% what is the abundance of the other two now to solve that it's very easy to go down the wrong path in terms of maths and start trying to do a quadratic equation and I really really wouldn't do that what I would do is I would sets out the equation as you would expect to set it out so we've got the abundance of magnesium 24 which we don't know what it is so we're going to call it X and we're going to multiply that by its mass which is 24 and we're going to add that to the magnesium 25 now we this is where you could go down the quadratic pathway by putting a Y in there and multiplying Y by 25 and then adding to that 10 times by 26 that's on the top axis but what you have to remember is of course that percentages add up to 100 and so if we've got ten percent of magnesium 26 and we've got X percent of magnesium 24 well the total is 100 and so this is going to be and we've used 10 and so this is going to be 90 takeaway X we still don't know what X is but we know that it is going to be whatever is left from 90 once you've taken X away is going to be our magnesium 25 so let me just draw that in quick pie chart so you can see what I mean so we've got 26 which is that section and we've got the rest which we don't know what it is but we know it is ninety percent and so if we have got 25 and 24 that's going to be X and this is going to be 90 takeaway X on that and then we go back to what we know from before because we're working with percentages so we need to divide all of this by 100 what's different this time of course is we know what our answer is so to speak 24 point three is the average on here and now we've set up this our final job is just to rearrange this and so we've got 24 point 3 multiplied by 100 is equal to 24 X plus 2,200 50 which is 90 x 25 takeaway 25 X plus 260 on that and then this of course gives us two thousand four hundred and thirty on the left-hand side and then this simplifies down to two thousand five hundred and ten takeaway X on there and so if we move X over to this side we've got x equals and then we take away the two thousand four hundred thirty from both sides we get two thousand five hundred and ten to eight two thousand four hundred and thirty which is 80% which means the percentage of magnesium 24 is eighty percent up here if the magnesium-26 is ten percent that gives us 90 percent which means our magnesium 25 is ten percent at the end so we've worked out our 200 two unknowns of 80 percent and 10 percent on there okay that's everything that's the whole of one topic in probably about an hour so well done everybody I hope that was useful and I'll see you again soon on another video thanks very much bye bye