hydrogen.

Most of sea water, therefore, is just water, that is, pure water. But it
contains some other substances as well and the best known of these is
salt. Salt is a substance the molecules of which contain atoms of sodium
and of chlorine. That is why sea water is about 1.1 percent sodium and
about 2.1 percent chlorine. There are some other kinds of atoms in sea
water, as you would expect, for it gets all the substances which the
waters of the earth dissolve and carry down to it but they are
unimportant in amounts.

Now we know something about the names of the important kinds of atoms
and can take up again the question of how they are formed by protons and
electrons. No matter what kind of atom we are dealing with we always
have a nucleus or center and some electrons playing around that nucleus
like tiny planets. The only differences between one kind of atom and any
other kind are differences in the nucleus and differences in the number
and arrangement of the planetary electrons which are playing about the
nucleus.

No matter what kind of atom we are considering there is always in it
just as many electrons as protons. For example, the iron atom is formed
by a nucleus and twenty-six electrons playing around it. The copper atom
has twenty-nine electrons as tiny planets to its nucleus. What does that
mean about its nucleus? That there are twenty-nine more protons in the
nucleus than there are electrons. Silver has even more planetary
electrons, for it has 47. Radium has 88 and the heaviest atom of all,
that of uranium, has 92.

We might use numbers for the different kinds of atoms instead of names
if we wanted to do so. We could describe any kind of atom by telling how
many planetary electrons there were in it. For example, hydrogen would
be number 1, helium number 2, lithium of which you perhaps never heard,
would be number 3, and so on. Oxygen is 8, sodium is 11, chlorine is 17,
iron 26, and copper 29. For each kind of atom there is a number. Let's
call that number its atomic number.

Now let's see what the atomic number tells us. Take copper, for example,
which is number 29. In each atom of copper there are 29 electrons
playing around the nucleus. The nucleus itself is a little inner group
of electrons and protons, but there are more protons than electrons in
it; twenty-nine more in fact. In an atom there is always an extra proton
in the nucleus for each planetary electron. That makes the total number
of proto