What's the Difference between Mass Number and Atomic Weight?
let's talk about
mass
number
andatomic
weight
these things can seem very similar but they're actually quite different so up here I got two atoms and I'm particularly interested in thenumber
of protons and neutrons that are in the nucleus of both these atoms so the protons are red the neutrons are in blue we could count them up the atom over here has five protons and five neutrons while the atom on this side also has five protons but has six neutrons somass
number
if as you may
remember is the
number
of protons plus thenumber
of neutrons so the atom over here has amass
number
of ten and the atom here has amass
number
of eleven five plus six okay now themass
number
also tells us a little bit about how much the atom actually weighs because it turns out that one proton or one Neutron weighs just about 1 amu oneatomic
mass
unit it's a unit that we can use to measure how much in atom weighs so if we have 10 protons and neutrons over here with amass
number
of 10that means that this atom is going to weigh about 10 am utan
atomic
mass
units and our atom over here with amass
number
of 11 is going to weigh about 11 a.m. you okay so that'smass
number
mass
number
is something that we can use to describe an atom it's protons plus neutrons and it tells us a little bit about how much that atom weighs inatomic
mass
units now let's move on to talk aboutatomic
weight
so you might notice here that both of these atoms have the samenumber
of protonswhich means that they are the same element right because the
number
of protons that an atom has in its nucleus determineswhat
element it is sowhat
element are these two atoms we can look on the periodic table to find that out and it turns out that boron with anatomic
number
of five iswhat
both of these atoms because they have five protons in their nucleus so these two atoms are both different versions of boron different versions of boron that have a differentnumber
of neutrons but the samenumber
of protons we call these isotopes okay isotopes are like different versions of an atom that have the samenumber
of protons but differ in theirnumber
of neutrons so this atom over here we call it boron 10 that's its isotope name because 10 themass
number
is 10 and this one over here is boron 11 because it has amass
number
of 11 so two versions of boron now there are billions and trillions gazillions of boron atoms in the world and if you picked out one boron it could be either of
these two versions okay it could be either a boron 10 atom with five neutrons or it could be a boron 11 atom with six neutrons but here's the thing there's not the same
number
of boron 10 and boron 11 atoms in the world okay if we picked out 50 random boron atoms just anywhere this iswhat
they look like okay the vast majority of them would be these Orange boron 11 atoms and a much smaller minority would be boron 10 which I'm representing with these these green circles here it turnsout that if we do the math that only about 20% of all of the boron atoms in the world are boron 10 with 5 neutrons and 80% the vast majority are boron 11 atoms we can look at
what
this looks like on a pie graph just to get a better idea of boron 11 taking up all this room as opposed to the small amount of boron 10 atoms so 20% of the atoms of boron are boron 10 80 percent of the atoms of boron are boron 11 this leads us right to the question that Thomas wait asks anatomic
weight
asks uswhat
isthe average
mass
of a boron so some of them weigh 10 a.m. you others weigh 11 a.m.what
is the averagemass
of a Borana now you might think average that's easy 10 a.m. you plus 11 a.m. you and I can divide by 2 but no that's wrong that's not the right way to find the averagemass
of a boron atom here's why that's wrong because this formula 10 plus 11 divided by 2 that assumes that we have the samenumber
of boron 10 in boron 11 if it were 50% this and 50% that then sureenough we could just add up the two
weight
s and divide by 2 but because we have only 20% of this in 80% of this we can't just add them up and divide by 2 we have to use a more complicated equation that takes into account the amount of each that we have here's how we do that here's how we would determine theatomic
weight
okay so boron 10 we have 20% of the total for on atoms or boron 10 so we're going to do 20% times 10 amu which iswhat
the boron 10 atom weighs and then
we're going to go over here and do 80% that's how much boron 11 we have times 11 a.m. you if you want you can put these in parentheses just so you can see the math a little bit easier when we end up multiplying this out we'll have to convert the percentages into decimals into 0.2 and 0.8 we end up with 10.8 amu that is the average
mass
of a boron atom when you take into account that there are only 20% 10 and 80% 11 so the averagemass
turns out to be a lot closer to 11 a.m. you thenit is 2:10 and that makes sense because there's so many more of these the average should be closer to 11 a.m. you than to 10 and here's the final thing now you know
what
thisnumber
is here at the bottom of these elements on the periodic table 10.8 this isn't amass
number
this is anatomic
weight
this tells you the averagemass
of a boron atom based on the fact that there are different amounts of the different isotopes so just to reviewmass
number
is something that applies to anindividual atom or to an individual isotope ok the
mass
number
of this is 10 because it has 5 protons and 5 neutrons themass
number
of this is 11 because it has 5 protons and 6 neutrons these are individual atoms or individual isotopesatomic
weight
takes these two isotopes into account and it also takes into account the amount of each that we have 20% of this 80% of this so we have to use these percentages when we do the math to figure outwhat
the averagemass
of all the different types ofboron would be so if you want to learn more about
