The Science Book

211

The gold foil experiment
In 1909, Rutherford set out to probe
the structure of matter using alpha
particles. The previous year, along
with the German Hans Geiger,
he had developed zinc sulphide
“scintillation screens,” which
enabled individual collisions of
alpha particles to be counted as
brief bright flashes, or scintillations.
With the help of undergraduate
student Ernest Marsden, Geiger
would use these screens to
determine whether matter was
infinitely divisible or whether
atoms contained fundamental
building blocks.
They fired a beam of alpha
particles from a radium source at
an extremely thin strip of gold leaf,
just a thousand or so atoms thick.
If, as the plum-pudding model held,
gold atoms consisted of a diffuse
cloud of positive charge with
pointlike negative charges, then the
massive, positively charged alpha
particles would plough straight

through the foil. Most of the

particles would be deflected only

slightly by interaction with the

gold atoms and would be scattered

across shallow angles.

Geiger and Marsden spent

long hours sat in the darkened

laboratory, peering down

It was quite the most

incredible event that has ever

happened to me in my life. It

was almost as incredible as if

you fired a 15-inch shell at a

piece of tissue paper and it

came back and hit you.

Ernest Rutherford

microscopes and counting the tiny

flashes of light on the scintillation

screens. Then, acting on a hunch,

Rutherford instructed them to

position screens that would catch

any high-angle deflections as

well as at the expected low-angle

scintillations. With these new

screens in place, they discovered

that some of the alpha particles

were being deflected by more than

90º, and others were rebounding off

the foil back the way they came.

Rutherford described the result as

like firing a 15-inch shell at tissue

paper and having it bounce back.

The nuclear atom

Halting heavy alpha particles in

their tracks or deflecting them by

high angles was possible only if

the positive charge and mass of

an atom were concentrated in

small volume. In light of these

results, in 1911, Rutherford

published his conception of

the structure of the atom. The

“Rutherford Model” is a solar

system in miniature, with electrons

orbiting a small, dense, positively

charged nucleus. The model’s

major innovation was the

infinitesimally small nucleus,

which forced the uncomfortable

conclusion that the atom is not at

all solid. Matter at an atomic scale

is mostly space, governed by

energy and force. This was a

definitive break from the atomic

theories of the previous century.

While Thomson’s “plum-

pudding” atom had been an

instant hit, Rutherford’s model was

largely ignored by the scientific

community. Its failings were all too

plain to see. It was well established

that accelerating electric charges

emit energy as electromagnetic

radiation. Thus, as electrons swoop

around the nucleus—experiencing

circular acceleration that keeps ❯❯

Geiger and Marsden aimed alpha particles from

a radioactive source at an incredibly thin gold leaf.

The scintillation screen could be spun around to

detect particles rebounding at any given angle.

Circular scintillation

screen

Thin gold

foil

Beam of

particles

Source of

particles

Scattered

particles

A PARADIGM SHIFT