Since radioactive atoms gave off either positively charged particles or
negatively charged particles, it seemed reasonable to assume that atoms
generally were made up of both types of electricity. Furthermore, since
the atoms in matter generally carried no charge at all, the normal
“neutral atom” must be made up of equal quantities of positive charge
and negative charge.
It turned out that only radioactive atoms, such as those of uranium and
thorium, gave off positively charged alpha particles. Many atoms,
however, that were not radioactive, could be made to give off electrons.
In 1899 Thomson showed that certain perfectly normal metals with no
trace of radioactivity gave off electrons when exposed to ultraviolet
light. (This is called the “photoelectric effect”.)
It was possible to suppose, then, that the main structure of the atom
was positively charged and generally immovable, and that there were also
present light electrons, which could easily be detached. Thomson had
suggested, as early as 1898, that the atom was a ball of matter carrying
a positive charge and that individual electrons were stuck throughout
its substance, like raisins in pound cake.
If something like the Thomson view were correct then the number of
electrons, each with one unit of negative electricity, would depend on
the total size of the positive charge carried by the atom. If the charge
were +5, there would have to be 5 electrons present to balance that. The
total charge would then be 0 and the atom as a whole would be
electrically neutral.
If, in such a case, an electron were removed, the atomic charge of +5
would be balanced by only 4 electrons with a total charge of -4. In that
case, the net charge of the atom as a whole would be +1. On the other
hand, if an extra electron were forced onto the atom, the charge of +5
would be balanced by 6 electrons with a total charge of -6, and the net
charge of the atom as a whole would be -1.
Such electrically charged atoms were called “ions” and their existence
had been suspected since Faraday’s day. Faraday had known that atoms had
to travel through a solution under the influence of an electric field to
account for the way in which metals and gases appeared at the cathode
and anode. It was he who first used the term, ion, from a Greek word
meaning “traveller”. The word had been suggested to him by the English
scholar, William Whewell (1794-1866). In 1884 the Swedish chemist Svante
August Arrhenius (1859-1927) had first worked out a detailed theory
based on the suggestion that these ions were atoms or groups of atoms
that carried an electric charge.
[Illustration: _Svante A. Arrhenius_]
By the close of the 19th century, then, Arrhenius’s suggestion seemed
correct. There were positive ions made up of atoms or groups of atoms,
from which one or more of the electrons within the atoms had been
removed. There were negative ions made up of single atoms or of groups
of atoms, to which one or more extra electrons had been added.
[Illustration: ]
Neutral atom
Each unit of positive charge is balanced by a unit of negative
charge
In this case, total charge = +2 -2 = 0
Ionized atom:
If an electron is removed, the balance is destroyed
In this case, total charge = +2 -1 = +1
Although Thomson’s model of the atom explained the existence of ions and
the fact that atoms could give off electrons or absorb them, it was not
satisfactory in all ways. Further investigations yielded results not
compatible with the raisins-in-the-pound-cake notion.
In 1906 Rutherford began to study what happened when massive subatomic
particles, such as alpha particles, passed through matter. When alpha
particles passed through a thin film of gold, for instance, they raced
through, for the most part, as though nothing were there. The alpha
particles seemed to push the light electrons aside and to act as though
the positively charged main body of the atom that Thomson had pictured
was not solid, but was soft and spongy.
The only trouble was that every once in a while an alpha particle seemed
to strike something in the gold film and bounce to one side. Sometimes
it even bounced directly backward. It was as though somewhere in each
atom there was something at least as massive as the alpha particle.
How large was this massive portion of the atom? It couldn’t be very
large for if it were the alpha particles would hit it frequently.
Instead, the alpha particles made very few hits. This meant the massive
portion was very small and that most alpha particles tore through the
atom without coming anywhere near it.
[Illustration: _Rutherford’s alpha particle bombardment apparatus. A
piece of radium in the lead box (B) emits alpha particles that go
through the gold foil (F). These particles are scattered at different
angles onto the fluorescent screen (S), where the flashes caused by each
impact are seen through the microscope (M). Below, alpha particles are
shown bouncing off a nucleus in the gold foil._]
[Illustration: ]
By 1911 Rutherford announced his results to the world. He suggested that
just about all the mass of the atom was concentrated into a very tiny,
positively charged “nucleus” at its center. The diameter of the nucleus
was only about 1/10,000 the diameter of the atom. All the rest of the
atom was filled with the very light electrons.
[Illustration: _Hans Geiger (left) and Ernest Rutherford at Manchester
University about 1910._]
According to Rutherford’s notion, the atom consisted of a single tiny
positively charged lead shot at the center of a foam of electrons. It
was Thomson’s notion in reverse. Still, the nucleus carried a positive
charge of a particular size and was balanced by negatively charged
electrons. Rutherford’s model of the atom explained the existence of
ions just as easily as Thomson’s did and it explained more besides.
For instance, if all the electrons are removed so that only the nucleus
remains, this nucleus is as massive as an atom but is so tiny in size
that it can penetrate matter. The alpha particle would be a bare atomic
nucleus from this point of view.
Rutherford’s model of the “nuclear atom” is still accepted today.