If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

# Average atomic mass

AP.Chem:
SPQ‑1 (EU)
,
SPQ‑1.A (LO)

## Video transcript

the thing that I've always found amazing about chemistry it's an entire field of science that we as human beings have developed to actually understand what is happening at an almost unimaginably small scale in particular we're going to be thinking about the atomic and even the sub atomic scale and by looking at that scale we can then begin to understand the universe in which we live in the scale in which we live in and even be able to make predictions about what will happen and make things that are useful for for human beings so if we're going to operate at this small of a scale and we're going to appreciate in a few seconds how small of a scale it is we're going to have to have some units of measurement and this video is going to focus on mass how do we measure mass at such a small scale well to do that the chemistry community has historically used something called an atomic mass unit right here atomic atomic mass unit and it's historically denoted as amu and more recently the more modern version of this is the unified atomic mass unit that is denoted by just a U instead of an amu so how does a unified atomic mass unit connect to our units of mass that we might use on a larger scale like say grams or kilograms well the unified atomic mass unit is defined as one point six six zero five four zero times 10 to the negative 27 kilograms so when you see something like this you might have a few reactions your first reaction which would be an appropriate reaction is that Wow 10 to the negative 27 power is very small to appreciate it you could write it out it would be zero point and then 26 zeroes and then you would have one six six zero five four zero so very very very small really unimaginably small we can only try to abstract it with things like mathematics the other thing you might appreciate is like this feels like a bit of a hairy number here one point six six zero five four zero why did they define it that way and the answer to your question is this definition makes it a lot cleaner when we think about the mass of whether it's an atom or the constituents of an atom like a proton or a neutron roughly speaking the mass of a proton is approximately one unified atomic mass unit the mass of a neutron is approximately one unified atomic mass unit it actually turns out that a proton is a little bit more than one it's about one point zero zero seven time ik mass units but it's approximately one and the neutron is actually a little bit more than even a proton it's one point zero zero eight approximately unified atomic mass units now no electrons mass is actually far smaller than either of these it's actually almost 1 mm of a proton or neutron and so you can imagine an atom which is made up of protons and usually neutrons and electrons as well the mass is mainly going to be the protons and neutrons in the nucleus and so if you know the number of protons and neutrons in the nucleus you're going to have a pretty good sense of its atomic mass and you can see that indicated on a periodic table of elements which we have here and we will study the periodic table of elements in a lot more detail in other videos but you can see a couple of interesting elements one you have the abbreviation of a given element H represents hydrogen the number on top on this periodic table that's the atomic number and that tells you how many protons it has and an element is defined by the number of protons so any atom that has exactly one proton in its nucleus is going to be hydrogen by definition any atom that has exactly 20 protons in its nucleus is going to be calcium by definition any atom that has exactly 36 protons in its nucleus is going to be Krypton by def finition so what would you expect the mass of a hydrogen atom to be pause this video and think about it well we know that all hydrogen atoms by definition have one proton but it actually turns out there's different versions of hydrogen that can have different numbers of neutrons most of the hydrogen in the universe actually has zero neutrons zero neutrons there are versions that have one or two neutrons but most 99.98% roughly of hydrogen in the universe has 1 proton 0 neutrons and if it's a neutral hydrogen it's going to have one electron and when we talk about versions of a given element there's a fancy word for it they're called isotopes and the different isotopes they'll all have the same number of protons because they're talking about the same element but they'll have different numbers of neutrons and so if this is the most common form of hydrogen what do you think its mass is going to be well its mass is going to be essentially the mass of a proton plus an electron and roughly speaking it's going to be the mass of a proton because the mass of a proton is going to be so much larger than the mass of an electron and so you would expect that its mass is approximately one unified atomic mass unit now if you were to precisely look at the mass of a proton and a electron if you add them together you actually get something that's a little bit closer to 1.008 and you actually see that right over here on the periodic table of elements now this number although it is pretty close to the mass of the version of hydrogen that I just described it's actually a weighted average of the various versions of hydrogen it's just close to this version because this version represents most of the hydrogen that we actually see around us if for example you had two versions of an element some hypothetical element and let's say that 80% of the element that we see is version 1 and version 1 has a mass of let's call it 5 atomic mass units and then version 2 the remainder 20% of what we observe of that element it has an atomic mass of six atomic mass units you would get a weighted average here of 5.2 unified atomic mass units and that's actually how these numbers are calculated they are not just the mass of one type of that element they are weighted average mass of the various isotopes of the various types and so this number on a periodic table of elements is known as the average atomic mass average average atomic atomic mass now in older chemistry books and this is actually the case when I first learned chemistry they call this number atomic weight and I've always complained about it because it's really talking about mass not weight if you don't know the difference you'll learn that at some point in the future it's really talking about average atomic mass now I'll give you one little detail that might be useful to you sometimes you'll hear something called relative atomic mass it actually turns out this periodic table of elements because it does not write a little you after each of these numbers it's essentially these numbers are unit lists so it's really talking about relative atomic mass so it's saying hey on average for example the mass of a carbon atom is going to be roughly 12 times that of on average the mass of a hydrogen atom if you they put the unit's here then that would actually truly be average atomic mass but for our purposes as we go into chemistry you can look at these numbers and say okay if oxygen has a relative atomic mass of 16 its average atomic mass is going to be 16 unified atomic mass units and as we will see in the future this understanding of average atomic mass will prove to be very very useful
AP® is a registered trademark of the College Board, which has not reviewed this resource.