This emphasis on the formulae of compounds
on the ratios in which atoms combine
are what Mendeleev knew as the valances of the elements,
lie at the heart of the periodic law.
Without it, the law would often seem ridiculous.
For example, look at these elements, silicon
and carbon.
They lie at the top of group 4.
One form of carbon is diamond.
So, when diamonds burn they form a gas.
As you can see, it's quite heavy.
Now, we could burned silicon in the same way,
but the product wouldn't have been a gas,
it would have been this.
This is the oxide of silicon.
It's some sand that I picked up on a Norfolk beach.
So, when carbon and silicon are burnt in oxygen,
the products look completely different,
they have completely different structures, too.
But, they both have the same kind of formula, they're both dioxides.
In both compounds,
there are two oxygen atoms for every carbon or silicon atom,
and it was this similarity in the formulae of the highest oxides
that led Mendeleev to put carbon and silicon in the same group.
