The Science Book

210 ERNEST RUTHERFORD

The plum-pudding model
Thomson’s discovery of the
electron earned him the Nobel
Prize for Physics in 1906. He was
enough of a theoretician, however,
to see that a radical new model
of the atom was needed to
adequately incorporate his
findings. His answer, produced
in 1904, was the “plum-pudding”
model. Atoms have no overall
electric charge and, since the
mass of this new electron was
small, Thomson postulated that a
larger positively charged sphere
contained most of the atom’s mass,
and the electrons were embedded
in it like plums in the dough of
a Christmas pudding. With no
evidence to suggest otherwise,
it was sensible to assume that
the point charges, like the plums
in a pudding, were arbitrarily
distributed across the atom.

Rutherford revolution
However, the positively charged
parts of the atom steadfastly
refused to reveal themselves, and
the hunt was on to locate the
missing member of the atomic
pair. The quest resulted in a
discovery that would produce a

very different visualization of the

internal structure of the basic unit

of all elements.

At the Physical Laboratories

at the University of Manchester,

Ernest Rutherford devised and

directed an experiment to test

Thomson’s plum-pudding model.

This charismatic New Zealander

was a gifted experimentalist with

a keen sense of which details to

pursue. Rutherford had received the

1908 Nobel Prize in Physics for his

“Theory of Atomic Disintegration.”

The theory proposed that

the radiations emanating from

radioactive elements were the

result of their atoms breaking apart.

With the chemist Frederick Soddy,

Rutherford had demonstrated that

radioactivity involved one element

spontaneously changing into

another. Their work was to suggest

new ways to probe the inside of

the atom and see what was there.

Radioactivity

Although radioactivity was first

encountered by Becquerel and

the Curies, it was Rutherford who

identified and named the three

different types of what we would

now call nuclear radiation. These

are slow-moving, heavy, positively

charged “alpha” particles; fast-

moving, negatively charged “beta”

particles; and highly energetic but

uncharged “gamma” radiation

(p.194). Rutherford classified these

different forms of radiation by

their penetrating power, from the

least-penetrating alpha particles,

which are blocked by thin paper,

to gamma rays that require a

thickness of lead to be stopped.

He was the first to use alpha

particles to explore the atomic

realm. He was also the first to

outline the notion of radioactive

half-life and discover that “alpha

particles” were helium nuclei—

atoms stripped of their electrons.

Ernest Rutherford Brought up in rural New Zealand,

Ernest Rutherford was working in

the fields when the letter from

J. J. Thomson arrived informing

him of a scholarship to Cambridge

University. In 1895, he was made a

research fellow at the Cavendish

Laboratories, where he conducted

experiments alongside Thomson

that led to the discovery of the

electron. In 1898, at 27 years old,

Rutherford took up a professorial

post at McGill University in

Montreal, Canada. It was there

that he carried out the work on

radioactivity that won him the

1908 Nobel Prize in Physics.

Rutherford was an accomplished

administrator, too, and during

his lifetime he headed up the

three top physics research

laboratories. In 1907, he took

the chair in physics at the

University of Manchester

where he discovered the atomic

nucleus. In 1919, he returned to

the Cavendish as director.

Key works

1902 The Cause and Nature

of Radioactivity, I & II

1909 The Nature of the α Particle

from Radioactive Substances

All science is either physics

or stamp collecting.

Ernest Rutherford