THE FUNDAMENTALS OF PHYSICSSCINTILLATING SCREENLARGE
DEFLECTIONRADIOACTIVE
SOURCEGOLD FOILBEAM DEFLECTEDBEAM OF ALPHA
PARTICLESBEAM TRANSMITTED
WITH LITTLE OR NO
DEFLECTION5 In Britain in the early 19th century,
John Dalton suggested that all atoms in
a given chemical element are exactly
alike: the atoms of different elements
being distinguished by their mass. He
had discovered that the weights of the
various elements involved in chemical
reactions were always in simple
numerical proportions. The simplest
example involved the gases, hydrogen
and oxygen, combining to make water.
Measurements showed that, if all of
the gases were to be used, the weight
of t he oxygen would need to be eight
times as much as t hat of hyd rogen. As
two hydrogen atoms and one oxygen
atom have combined to ma ke a
molecule of water (H 2 O) it implies that
one oxygen atom must weigh eight
times as much as two atoms of
hyd rogen. So a n atom of oxygen is 16
times heavier than one of hydrogen.In his Manchester laboratory, the physicist Ernest Rutherford
and his colleagues found a way to probe the heart of an atomTHE KEY EXPERIMENT
By studying ma ny chemical
reactions and measuring the relative
amounts of the elements involved,
by the mid-19th century the relative
masses of their basic atoms had been
determined. Relative to hydrogen,
atoms of oxygen, carbon, calcium and
iron weighed 16, 12, 40 and 56 times
as much. This tantalising numerology
was a hint that atoms of different
elements may share some commonAt Manchester University,
Ernest Rutherford led
an experiment into
alpha-particle scattering
at large anglesEarly in the 20th century, Ernest
Rutherford noticed that thin sheets
of mica could deflect alpha particles
(see ‘Need to know’, p37), which were
moving at 15,000km/s. This could only
have happened if they felt electric and
magnetic forces far greater than anything
known. He mused that these forces might
be present within atoms. Rutherford
suggested that his colleague, Ernest
Marsden, look to see if any alpha particleswere deflected through very large angles.
Instead of mica, Marsden used gold leaf
that was only a few hundred atoms thick,
and a scintillating screen to detect the
scattered alpha particles. To everyone’s
amazement, he discovered that about one
in 20,000 alphas were turned back in their
tracks. Rutherford famously exclaimed:
“It was as though you had fired a 15-inch
shell at a piece of tissue paper and it had
bounced back and hit you.”Rutherford realised that the positive
charge in an atom is concentrated in a
massive and exceedingly compact central
‘nucleus’, and that it was the repulsion
of like charges that was deflecting the
relatively lightweight alpha (the nucleus
of a gold atom being some 50 times more
massive than an alpha particle).
The size of the nucleus relative to an
atom was famously compared to being
like a “fly in a cathedral”.