How can you calculate the mass of an atom?

As you have probably learned, an atom is a very, very small object.

However, we can easily handle a very

large group of atoms and weigh that. If we have an idea of how to

figure out how many atoms are in that particular group, we can

simply divide the mass of the group into the individual atoms in

that group. For example, let us say for the moment that we know that

there are 1000 atoms of carbon for every 1g of charcoal, if I get

the mass of a piece of charcoal (which is pure carbon) and it turns

out to be 2g in mass, then I know that there are 2000 atoms of carbon in there.

The tricky part is knowing how many atoms of carbon are in 1g of

charcoal. Here is the problem, as you already noted, we can say that

1 oxygen atom is equivalent in mass to 16 hydrogen atoms, this would

be good if we knew what the mass was of either oxygen or hydrogen.

Let us say, we did not, but we knew that 12 atoms of hydrogen were

equivalent to the mass of 1 carbon atom, then if we knew the mass of

carbon, then we could know the mass of hydrogen and oxygen. But let

us say we did not know the actual mass of any atom, what then? All

we have are relative masses, the mass of an atom in relation to

another atom. What we need is a starting point.

Scientists simply agreed that 12g of carbon atoms would contain a

"mole" of carbon atoms. At this point, it does not matter how many

atoms are in a mole, let us say for the moment that it turns out

that there are 12 atoms in a mole (a mole in this case is just like

"dozen") then each atom of carbon must have a mass of 1g (12g

divided by 12atoms). Then we can say that the mass of a hydrogen is

1/12g (since 1 carbon is to 12 hydrogen) and the mass of oxygen is

16/12g (or 4/3g, since 16 hydrogens is 1 oxygen). The point here is

that once we agree on a grouping of atoms (the mole) and agree on

the mass of that grouping for one of the atoms, then we would know

the mass of all the other atoms. Once we have our starting point, we

know all the relative masses.