Good question. Nineteenth century chemists couldn't take
individual atoms and plunk them on a scale. They could only study
chemical reactions involving huge numbers of atoms all at once.
So how would they ever find the weight of one atom by itself?
So that lets you calculate the weight of one atom? I don't quite
Think about what's going on in the reaction: two hydrogen atoms and one oxygen
atom join together to make one water molecule. So if you want to end up with
nothing but water molecules, you have to start with atoms in the right
proportion: two hydrogens for every oxygen.
I see! Then one oxygen atom must weigh eight times as much as two
Exactly. Mendeleev and his contemporaries couldn't say how much a given
atom weighed, in pounds or grams, but, by
studying reactions in this way, they could tell you how heavy it was in
relation to other atoms. One oxygen atom weighs as much as sixteen
hydrogens; carbon, twelve hydrogens; helium, four, and so on. Thus
Mendeleev was able to arrange the elements in order from lightest to