The Science Book

212 ERNEST RUTHERFORD

them in their orbits—they
ought to be continually emitting
electromagnetic radiation. Steadily
losing energy as they orbited, the
electrons would spiral inexorably
into the nucleus. According to
Rutherford’s model, atoms ought to
be unstable, but clearly they are not.

A quantum atom
Danish physicist Niels Bohr saved
the Rutherford model of the atom
from languishing in obscurity
by applying new ideas about
quantization to matter. The
quantum revolution had begun
in 1900 when Max Planck had
proposed the quantization of
radiation, but the field was still
in its infancy in 1913—it would
have to wait until the 1920s for a
formalized mathematical framework
of quantum mechanics. At the
time Bohr was working on
this problem, quantum theory
essentially consisted of no more
than Einstein’s notion that light
comes in tiny “quanta” (discrete
packets of energy) that we

falling out of orbit into the nucleus

was, for electrons, impossible.

Bohr’s was a purely theoretical

model of the atom. However, it

agreed with experiment and solved

many associated problems in an

elegant stroke. The way in which

electrons would have to fill up

empty shells in a strict order,

getting progressively farther from

the nucleus, matched the march

of the properties of the elements

seen across the periodic table as

atomic number increases. Even

now call photons. Bohr sought to

explain the precise pattern of

absorption and emission of light

from atoms. He suggested that

each electron is confined to fixed

orbits within atomic “shells,” and

that the energy levels of the orbits

are “quantized”—that is, they can

only take certain specific values.

In this orbital model, the

energy of any individual electron

is closely related to its proximity to

the atom’s nucleus. The closer an

electron is to the nucleus, the less

energy it has, but it can be excited

into higher energy levels by

absorbing electromagnetic

radiation of a certain wavelength.

Upon absorbing light, an electron

leaps to a “higher,” or outer, orbit.

Upon attaining this higher state,

the electron will promptly drop

back into the lower-energy orbit,

releasing a quantum of energy that

precisely matches the energy gap

between the two orbitals.

Bohr offered no explanation for

what this meant or what it might

look like—he simply stated that

If your experiment needs

statistics, you ought to have

done a better experiment.

Ernest Rutherford

The plum-pudding model of the atom
with the electrons spread across a diffuse
nucleus was replaced by Rutherford’s
model with electrons in orbit around
a small, dense nucleus. Bohr refined
Rutherford’s model by adding
quantized orbits for the electrons.
Here, a carbon atom is illustrated.

Plum-pudding model Rutherford model Bohr model

Electron Proton Neutron

6 protons +

6 neutrons