As long ago as ancient Greek times, there were men who suspected that
all matter consisted of tiny particles which were far too small to see.
Under ordinary circumstances, they could not be divided into anything
smaller, and they were called “atoms” from a Greek word meaning
“indivisible”.
It was not until 1808, however, that this “atomic theory” was really put
on a firm foundation. In that year the English chemist John Dalton
(1766-1844) published a book in which he discussed atoms in detail.
Every element, he suggested, was made up of its own type of atoms. The
atoms of one element were different from the atoms of every other
element. The chief difference between the various atoms lay in their
mass, or weight.[1]
Dalton was the first to try to determine what these masses might be. He
could not work out the actual masses in ounces or grams, for atoms were
far too tiny to weigh with any of his instruments. He could, however,
determine their relative weights; that is, how much more massive one
kind of atom might be than another.
For instance, he found that a quantity of hydrogen gas invariably
combined with eight times its own mass of oxygen gas to form water. He
guessed that water consisted of combinations of 1 atom of hydrogen with
1 atom of oxygen. (A combination of atoms is called a “molecule” from a
Greek word meaning “a small mass”, and so hydrogen and oxygen atoms can
be said to combine to form a “water molecule”.)
[Illustration: _John Dalton_]
To account for the difference in the masses of the combining gases,
Dalton decided that the oxygen atom was eight times as massive as the
hydrogen atom. If he set the mass of the hydrogen atom at 1 (just for
convenience) then the mass of the oxygen atom ought to be set at 8.
These comparative, or relative, numbers were said to be “atomic
weights”, so that what Dalton was suggesting was that the atomic weight
of hydrogen was 1 and the atomic weight of oxygen was 8. By noting the
quantity of other elements that combined with a fixed mass of oxygen or
of hydrogen, Dalton could work out the atomic weights of these elements
as well.
Dalton’s idea was right, but his details were wrong in some cases. For
instance, on closer examination it turned out that the water molecule
was composed of 1 oxygen atom and 2 hydrogen atoms. For this reason, the
water molecule may be written H₂O, where H is the chemical symbol for a
hydrogen atom, and O for an oxygen atom.
It is still a fact that a quantity of hydrogen combines with eight times
its mass of oxygen, so the single oxygen atom must be eight times as
massive as the 2 hydrogen atoms taken together. The oxygen atom must
therefore be sixteen times as massive as a single hydrogen atom. If the
atomic weight of hydrogen is 1, then the atomic weight of oxygen is 16.
At first it seemed that the atomic weights of the various elements were
whole numbers and that hydrogen was the lightest one. It made particular
sense, then, to consider the atomic weight of hydrogen as 1, because
that made all the other atomic weights as small as possible and
therefore easy to handle.
The Swedish chemist Jöns Jakob Berzelius (1779-1848) continued Dalton’s
work and found that elements did not combine in quite such simple
ratios. A given quantity of hydrogen actually combined with a little bit
less than eight times its mass of oxygen. Therefore if the atomic weight
of hydrogen were considered to be 1, the atomic weight of oxygen would
have to be not 16, but 15.87.
[Illustration: _Jöns Jakob Berzelius_]
As it happens, oxygen combines with more elements (and more easily) than
hydrogen does. The ratio of its atomic weight to that of other elements
is also more often a whole number. In working out the atomic weight of
elements it was therefore more convenient to set the atomic weight of
oxygen at a whole number than that of hydrogen. Berzelius did this, for
instance, in the table of atomic weights he published in 1828. At first
he called the atomic weight of oxygen 100. Then he decided to make the
atomic weights as small as possible, without allowing any atomic weight
to be less than 1. For that reason, he set the atomic weight of oxygen
at exactly 16 and in that case, the atomic weight of hydrogen had to be
placed just a trifle higher than 1. The atomic weight of hydrogen became
1.008. This system was retained for nearly a century and a half.
Throughout the 19th century, chemists kept on working out atomic weights
more and more carefully. By the start of the 20th century, most elements
had their atomic weights worked out to two decimal places, sometimes
three.
A number of elements had atomic weights that were nearly whole numbers
on the “oxygen = 16” standard. The atomic weight of aluminum was just
about 27, that of calcium almost 40, that of carbon almost 12, that of
gold almost 197, and so on.
On the other hand, some elements had atomic weights that were far
removed from whole numbers. The atomic weight of chlorine was close to
35.5, that of copper to 63.5, that of iron to 55.8, that of silver to
107.9, and so on.
Throughout the 19th century, chemists did not know why so many atomic
weights were whole numbers, while others weren’t. They simply made their
measurements and recorded what they found. For an explanation, they had
to wait for a line of investigation into electricity to come to
fruition.