Why is the atomic mass of an element a weighted average?

Answer 1

This is because of the existence of isotopes. What's an isotope? It's an element that has a specific number of neutrons. I'll give an example. Carbon has 6 Neutrons, 6 Protons, 6 Electrons.

That's the most common form of Carbon. But 1% of all Carbon is an isotope that has 1 extra Neutron. So it weighs 1 more, also. So this isotope weighs 13 instead of 12. We call it Carbon 13, or 13C.

There's also a 14C and so on....but in far smaller amounts.

So anyhow, since we have 99% Carbon weighing 12 and 1% Carbon weighing 13, what weight do we put on our Periodic Table?

Well, I guess we could put the most common.

But what about Bromine? That has 2 major isotopes. One weighs 79 and the other 81, they are 51% and 49% (very roughly) abundant respectively.

That's almost an exact split...it'd be pretty ridiculous to only put the weight of one.

So instead we say it's weight is 79.9...which is it's weight averaged over all of it's isotopes, with respect to their abundance.

It doesn't seem right to average it on an atom-to-atom basis. But if I'm weighing 2grams of my sample, there are HUGE numbers of molecules there. And the only way I can do an accurate calculation is to know an averaged mass, because my sample WILL contain all of the isotopes.

If I calculate for only 1 isotopic weight, my calculations will be wrong. That's why we average them.

Answer 2

This is because of the existence of isotopes. What's an isotope? It's an element that has a specific number of neutrons. I'll give an example. Carbon has 6 Neutrons, 6 Protons, 6 Electrons.

That's the most common form of Carbon. But 1% of all Carbon is an isotope that has 1 extra Neutron. So it weighs 1 more, also. So this isotope weighs 13 instead of 12. We call it Carbon 13, or 13C.

There's also a 14C and so on....but in far smaller amounts.

So anyhow, since we have 99% Carbon weighing 12 and 1% Carbon weighing 13, what weight do we put on our Periodic Table?

Well, I guess we could put the most common.

But what about Bromine? That has 2 major isotopes. One weighs 79 and the other 81, they are 51% and 49% (very roughly) abundant respectively.

That's almost an exact split...it'd be pretty ridiculous to only put the weight of one.

So instead we say it's weight is 79.9...which is it's weight averaged over all of it's isotopes, with respect to their abundance.

It doesn't seem right to average it on an atom-to-atom basis. But if I'm weighing 2grams of my sample, there are HUGE numbers of molecules there. And the only way I can do an accurate calculation is to know an averaged mass, because my sample WILL contain all of the isotopes.

If I calculate for only 1 isotopic weight, my calculations will be wrong. That's why we average them.

Answer 3

This is because of the existence of isotopes. What's an isotope? It's an element that has a specific number of neutrons. I'll give an example. Carbon has 6 Neutrons, 6 Protons, 6 Electrons.

That's the most common form of Carbon. But 1% of all Carbon is an isotope that has 1 extra Neutron. So it weighs 1 more, also. So this isotope weighs 13 instead of 12. We call it Carbon 13, or 13C.

There's also a 14C and so on....but in far smaller amounts.

So anyhow, since we have 99% Carbon weighing 12 and 1% Carbon weighing 13, what weight do we put on our Periodic Table?

Well, I guess we could put the most common.

But what about Bromine? That has 2 major isotopes. One weighs 79 and the other 81, they are 51% and 49% (very roughly) abundant respectively.

That's almost an exact split...it'd be pretty ridiculous to only put the weight of one.

So instead we say it's weight is 79.9...which is it's weight averaged over all of it's isotopes, with respect to their abundance.

It doesn't seem right to average it on an atom-to-atom basis. But if I'm weighing 2grams of my sample, there are HUGE numbers of molecules there. And the only way I can do an accurate calculation is to know an averaged mass, because my sample WILL contain all of the isotopes.

If I calculate for only 1 isotopic weight, my calculations will be wrong. That's why we average them.