welcome to getchemistryhelp decom my name is Dr Kent and in this lesson we're going to discuss the atomic number mass number and Isotopes for different kinds of elements now elements differ by the number of protons they contain in their nucleus so the number of protons is what gives an element its identity so all atoms of an element always have the same number of protons so if you change the number of protons now You' transmuted the element into a different element so that number of protons is called the atomic number or Z and you can always find it written right above the elemental symbol on a periodic table so for example hydrogen this number one that's the atomic number for hydrogen so that tells me that hydrogen must have one proton or for example vadium okay 23 that tells me vadium must have 23 protons or let's say sulfur 16 okay well sulfur must have have 16 protons so again every atom of sulfur always has 16 protons that's what makes sulfur sulfur is the number of protons we can change the neutrons we can change the electrons but if you change the number of protons well now you've transmuted that into a different element for example if sulfur somehow gained one proton it would no longer be sulfur now it would become chlorine or sulfur lost a proton now it would become phosphorus now there's no way for us to trans mute elements or to add or gain protons in chemical reactions for that to occur we have to go into what's called nuclear chemistry which is a subject for a whole another lesson so that's the atomic number well the mass number is the number of protons and neutrons combined now it's called the mass number because you might recall from our lesson on atomic structure that the biggest particles the ones that have the most Mass are the protons and the neutrons and the electrons weigh about 1,800 times less so 99.9% of the mass comes from the protons and the neutrons which is why adding those together gives us what's called the mass number now the mass number is not the same as the atomic mass for example hydrogen this number right here is the atomic mass that is the actual average mass of a hydrogen atom so if you were to weigh out a hydrogen atom it would have an average mass or an atomic mass of 1.01 AMU there's a whole another lesson dedicated to this where I show you how to calculate this but this is not the mass number the mass number is always going to be some whole number like one or two or three it'll never be some kind of a decimal so the way we specify all of these atomic numbers and mass numbers is using what's called Atomic notation it's pretty straightforward so we'll just write the atomic symbol so for example if it was carbon I would put a big Capital C here now Z is the atomic number which is the number of protons so if I already to look on my periodic table I would see written right above carbon is the number number six so I would write six here so six protons and the mass number that's the protons and neutrons so carbon can have different numbers of neutrons so it might be carbon let's say 13 so that would be 13 protons and neutrons now you will see sometimes we don't always write this six on the bottom because it's a bit redundant because if we know that the element is carbon well again carbon always has six protons so there's no real need to write 6 in here so we might just write it as C with the superscript 13 here so let's look at an example here's the atomic notation for an isotope so the SI well SI tells me the element must be silicon now this number 14 well that's the atomic number so that's the number of protons so we must have 14 protons how about this 29 up here what is this well that is the mass number remember that that is the number of the protons combined with the neutrons so the mass number is 29 well if we've got 29 protons and neutrons and 14 protons how many neutrons would we have well I could just subtract the protons 14 from the number of protons and neutrons 29 so I must have 15 neutrons now I mentioned earlier that all atoms of the same element have the same number of protons in other words they all have the same atomic number but you can change the number of neutrons or the mass number so Isotopes are elements that have the same atomic number but now we've changed the number of neutrons we're given them different Mass numbers so there are many elements that have multiple Isotopes for example hydrogen has three common Isotopes one of them is hydrogen one so hydrogen if you look at on the periodic table it's got one protons so here's my one proton this number on top is the protons and neutrons well if it's got one proton it must have zero neutrons well another isotope of hydrogen is hydrogen 2 again it's still hydrogen so it still has to have one proton because that's what makes hydrogen hydrogen but this two tells me oh it must have gained a neutron here and the third isotope of hydrogen is hydrogen 3 again it's still hydrogen so they all have one proton but what makes them different isotopes is their neutrons are different so this one has zero neutrons this one has one Neutron and this one has two neutrons which gives this one a mass number 1 plus 0 1 gives this one a mass number of 1 plus one 2 this one a mass number 1 + 2 is three so again Isotopes all have the same atomic number or protons they differ by the number of neutrons now the way we can commonly refer to to these or name these Isotopes is just by given the name of the element followed by that mass number so for example if I were to say Cobalt 60 well that tells me it's Cobalt with a mass number of 60 again it's the mass number not the atomic number if I said carbon 14 oh well carbon with a mass number of 14 based on that how many protons and neutrons could you determine an atom of lead 206 has okay well lead so lead has the symbol PB so I look around here oh here we go right here is lead okay well lead is number 82 so that tells me lead must have 82 protons so what's this number 206 again that's the mass number that is the number of protons and neutrons added together so how many neutrons would I have well if I take 206 protons and neutrons and I subtract ract out the 82 protons I must have 124 neutrons in this particular isotope of lead 206 and again I'll just note that 206 is the mass number right that's always a whole number that is not the atomic mass 207.200 Boron so I find Boron is right here so what's the atomic number well again that's the number that's written above it five so the atomic number of boron is five that tells me Boron must have five protons what's the atomic mass of silicon okay so I find silicon here it is now again the atomic mass is not the mass number that's this number down here so silicon has an average atomic mass of 28.0 9 a muu and again you can refer to our lesson on atomic mass for a little more information on that how many electrons would a neutral neon atom have okay well let's find neon it's number 10 over here now it tells me it's neutral so what does that mean well that means it's not positive it's not negative it's just completely neutral well neon normally has 10 protons because it has an atomic number of 10 well in order for it to be neutral how many negative electrons must it have well it must have the exact same number so if it's got 10 protons that means it must have 10 electrons for it to be a neutral neon atom how about an atom that had 27 protons 32 neutrons and 27 electrons would that be neutral well let's see so 27 protons so 27 positives 32 neutrons 32 neutrals okay so those won't even affect the charge and then 27 electrons so 27 negatives so in order for it to be neutral what has to happen well the number of positives has to equal the number of negatives and they do so yes this would be electrically neutral and just for fun Which element would this be well we look here at the number of protons right it's got 27 protons so it must be element 27 it must be Cobalt how about a sodium atom with 10 electrons would that be electrically neutral so let's find sodium na it's right here okay well it's number 11 so that means it has 11 protons well if it only has 10 electrons are those going to cancel out and make it neutral well of course not in order for it to be neutral it would need 11 protons and 11 electrons so in this case it's got one extra proton so this would actually be sodium with a positive charge how many protons and neutrons does nitrogen 15 have have okay so nitrogen 15 so that tells me it's got 15 protons and neutrons together well let's find nitrogen it's right over here so how many protons does nitrogen have well again it's this number written right above it so it must have seven protons how many neutrons well I can just subtract the atomic number from the mass number so it must have eight neutrons how many neutrons are in nickel 61 remember it's named by the symbol and then the mass number okay neutrons so nickel 61 that tells me it's got 61 protons and neutrons how many protons must it have well I got to find it here on the perodic table there it is so it has 28 protons so I subtract 28 protons which means it must have 33 neutrons so there are 33 neutrons in nickel 61 and last one how could you name this isotope remember we name it by the element followed by the mass number so this case I would call it U as uranium so uranium and which isotope of uranium is it well it's uranium 238 so uranium hyphen 238 well I hope you enjoyed that lesson on atomic number mass number and Isotopes if you did be sure and leave me a comment down below and hit that like button and share the video with a friend for even more chemistry video lessons and practice problems be sure and visit my website at getchemistryhelp doccom thank you