The Science Book

A PARADIGM SHIFT 213

more convincing was the way in
which the theoretical energy levels
of the shells neatly fit actual
“spectral series”—the frequencies
of light absorbed and emitted by
different atoms. A long sought after
way to marry electromagnetism
and matter had been realized.

Going inside the nucleus
Once this picture of the nuclear
atom had been accepted, the next
stage was to ask what lay inside
the nucleus. In experiments
reported in 1919, Rutherford found
that his beams of alpha particles
could generate hydrogen nuclei
from many different elements.
Hydrogen had long been
recognized as the simplest of all
the elements and thought of as
a building block for all other
elements, so Rutherford proposed
that the hydrogen nucleus was in
fact its own fundamental particle,
the proton.
The next development in atomic
structure was James Chadwick’s
1932 discovery of the neutron, in
which Rutherford once again had
a hand. Rutherford had postulated
the existence of the neutron in 1920
as a way to compensate for the
repulsive effect of many point-sized

positive charges crammed into a

tiny nucleus. Like charges repel

each other, so he theorized that

there must be another particle

that somehow dissipates the

charge or binds the jostling protons

tightly together. There was also

extra mass in elements heavier

than hydrogen, which could be

accounted for by a third, neutral but

equally massive subatomic particle.

However, the neutron proved

difficult to detect and it took nearly

a decade of searching to find it.

Chadwick was working at the

Cavendish Laboratory under the

supervision of Rutherford. Guided

by his mentor, he was studying a

new kind of radiation that had been

found by the German physicists

Walther Bothe and Herbert Becker

when they bombarded beryllium

with alpha particles.

Chadwick duplicated the

Germans’ results and realized

that this penetrating radiation was

the neutron Rutherford had been

looking for. A neutral particle,

such as the neutron, is much

more penetrating than a charged

particle, such as a proton, because

it feels no repulsion as it passes

through matter. However, with

mass slightly greater than a proton,

it can easily knock protons out

of the nucleus, something that

otherwise only extremely energetic

electromagnetic radiation can do.

Electron clouds

The discovery of the neutron

completed the picture of the

atom as a massive nucleus

with electrons in orbit around

it. New discoveries in quantum

physics would refine our view of

electrons in orbit around a nucleus.

Modern models of the atom feature

“clouds” of electrons, which

represent only those areas in

which we are most likely to find

an electron, according to its

quantum wavefunction (p.256).

The picture has been further

complicated by the discovery

that neutrons and protons are

not fundamental particles, but

are made of arrangements of

smaller particles called quarks.

Questions about the true structure

of the atom are still actively

being researched. ■

James Chadwick discovered the neutron by bombarding

beryllium with alpha particles from radioactive polonium.

The alpha particles knocked neutrons out of the beryllium.

Then the neutrons dislodged protons from a layer of paraffin,

and these protons were detected by an ionization chamber.

Neutrons

Protons

Polonium

Ionization

Beryllium Paraffin chamber

The difficulties disappear

if it be assumed that the

radiation consists of particles

of mass 1 and charge 0,

or neutrons.

James Chadwick

Alpha

particles