all right now we're going to focus on one form of matter right off the bat here and that's going to be the atom and we're going to talk about what makes an atom up and what defines an atom atoms are made of three primary subatomic particles the electron the proton and the neutron these subatomic particles are differentiated on the basis of two factors mass and charge all right the first subatomic particle we want to talk about is the electron the electron is said to have negligible mass notice we did not say we use the term negligible mass not no mass electrons do have mass however their mass is so so small compared to that the mass of the proton and the neutron that when you add the mass of the electron to the mass of the proton you get the mass of the proton back because the mass of the electron is so small compared to the mass of the proton so we say electrons have a negligible mass when it comes to determine the weight of an atom or the mass of an atom we say that electrons have a relative charge of negative one before the end of the semester we'll actually define in terms of coulombs what the charge of an electron is but for for talking purposes for conceptual purposes we always refer to an electron as having a charge of negative one the proton is almost the bizarro evil twin of an electron it has a mass of one amu and a relative charge of plus one so what we mean is that a proton has a charge equal in magnitude but opposite in sine to that of an electron we say that a proton has a mass of one amu well what's an amu an amu stands for atomic mass unit and the atomic mass unit is defined as 1 12 the mass of a carbon-12 atom however you don't need to know that what you need to know is that an atomic mass unit is a unit of mass made up by chemists for the sole purpose of giving a proton a mass of 1 amu our final subatomic particle that we want to talk about is the neutron the neutron is a political moderate it wants to be taken seriously and given all the weight of other politicians but it's scared to take a stand so a neutron has a mass roughly equal to that of a proton a mass of one amu however it has no charge this also brings up one of my favorite jokes a neutron walks into a bar where the protons are bartender the neutron asks how much for a drink the proton says for you no charge the neutron says are you sure the proton says why yes i'm positive make sure you try the salad bar i'll be here all week one last thing i do want to point out is i remember how i said that the proton has significantly more mass than that of an electron a proton's mass is about 2 000 times the mass of an electron that's why when we're calculating the mass of an atom we really only look at the protons and neutrons and ignore the electrons contribution all right write down the word aiden a i d a n write down the word aiden a i d a n aiden it's a name right once you've written down the word aiden let's talk about electrons the electron was discovered by j.j thompson he discovered electrons while playing with a cathode ray tube this picture shows a picture of a cathode ray tube and i actually have a video either posted if i'm lucky edited into this lecture if i'm unlucky it's posted at the end here of a cathode ray tube a cathode ray tube consists of two metal plates these two metal plates are placed in a vacuum tube what do i mean by a vacuum tube i mean that all the air has been removed from this tube instead a phosphorescent screen has been placed in there in other words something that's going to show green when a cathode ray hits it the cathode ray tube is then hooked up to a power source the negative end of this power source is the metal plate known as the cathode the positive end of the electrical source is attached to the metal plate known as the anode when a charge is run through the tube a stream of particles is emitted from the cathode and travels to the anode so particles leave here and go to here because these particles leave the negative electrode or the cathode the particles are called referred to as cathode rays thompson did not invent the cathode ray tube he was simply playing with it and made some useful observations about it that led to the discovery of the electron what thompson noticed was a couple of things first thompson noticed that it didn't matter what he made the electrodes of no matter what he made these electrodes of he always saw cathode rays produced and those cathode rays always flowed from the cathode to the anode that let him conclude a couple important things he tried several different materials to make these cathodes from and no matter what material he used he always saw the electro he always saw the cathode rays flowing from the cathode to the anode that led him to conclude two things from that observation first all substances had to include these cathode rays also if the cathode rays always went from the negative to the positive the cathode rays had to have a negative charge why well they're flowing towards the positive charge right and opposites attract so if something's flowing towards the positive charge it itself must have a negative charge so those are two important observations thompson made the next observation was really awesome and allowed him to do some math he decided to place a magnetic field next to his cathode ray tube when he placed a magnetic field to that cath next to that cathode ray he noticed that the cathode rays were deflected he placed one end of the magnet on near the cathode rays and the cathode rays would actually bend towards the magnet if he then flipped the magnet over and used the other side of the magnet the electrons were actually repulsed by it or the cathode rays and yes of course i kind of let the cat out of the bags numerous time here times here this caso cathode ray is actually a stream of electrons so you discover that these streams stream of electrons definitely had a charge because they were responding to the presence of a magnetic field they did the same thing in the presence of an electrical field now the reason this was important was it was well known at the time the relationship between charge mass and force so what thompson did was he took electrical plates placed them on the side of this cathode ray tube here and he turned on these electrical plates to a certain voltage he then measured the degree to which he was deflecting the electrons as a function of char as a function of the potential of the electrical field that allowed him since he was essentially knew how much force he was applying he knew the mass to she was able to calculate the mass to charge ratio that these particles or these electrons had to have so thompson was able to calculate so by using that electrical field and measuring deflection thompson was able to calculate the mass to charge ratio of electrons however he was never able to either calculate the charge by itself of the electrons or the mass of the of itself by the electrons therefore he hit kind of a roadblock he knew that electrons existed he knew that all materials contained them and he was even able to determine their mass to charge ratio but he couldn't break the ratio deadlock he couldn't determine either the charge or the mass that was done by another chemist named millikan before we look at milken's work real quickly summarizing what thompson did he observed three things cathode rays always flowed from cathode to anode cathode arrays are deflected by negative electrodes the property of a cathode rays were independent of the material from which he made electra made the electrodes so he was able to conclude that cathode rays had a negative charge and that all matter must contain these negatively charged particles he was also able to calculate the mass to charge ratio but never them separately milken designed a very simple but elegant experiment to unravel the mass to charge ratio of an electron that thompson had calculated what milken did was used an aspirator to generate really really small oil drops let's look at this on a bigger picture here he used an aspirator to generate really really small oil drops he then allowed these oil drops to fall until they reached terminal velocity why this is important is because that helped him calculate the mass of these oil drops that he was creating what he then did was he then shot x-rays at these oil drops what these x-rays did was they kicked electrons off the oil drop so i've got my oil drop here i'm falling at a terminal velocity so i'm so milk is able to calculate my mass here milk comes along and says i'm shooting you with x-rays oh no and when the electron and when the oil drops are hit by an x-ray they lost an electron or multiple electrons that left the oil drop with a positive charge these positively charged oil drops are then falling between two charged plates using observation of the human eye he would manipulate the charge on the two plates until the charge of the two plates was enough to counteract the falling of the drop and freeze the drop in midair because he knew the voltage between the two plates and he knew the force due to the pull of gravity on each drop he was out able to calculate what the charge on each drop was so his experimental design was brilliant but what he did next i think was even more awesome now what milking couldn't control with his experiment was how many electrons were lost by each drop he could just calculate the charge in the drops some drops would lose two electrons some drops would lose three electrons some four some five some six so what milken did was he took the charges and all the drops and he subtracted the charges from one drop to this from he subtracted the difference in charges between all the drops and he found the smallest difference that he found between any drops and he assumed that that was the charge of an electron then to test it he divided the charges and all those drops by that smallest difference and if that was truly the charge of an electron then all those charges he should be seeing should be whole number multiples of that smallest amount which they were and thus became the charge on the electron and he discovered that the charge of an electron according to his measurements was 1.59 times 10 to the negative 19 coulombs further scientific work with more advanced equipment has come up with the current accepted value of 1.602 times 10 to the negative 19 coulombs coulombs for the charge of your electron why don't you write down sally write down the word sally sally malik you don't need to write down malek but write down sally s-a-l-l-y that's it