Rutherford’s Gold
Foil Experiment
The History of the Atom
• Early philosophers
considered that all
matter was made from
one or more earth
elements:
Air
Earth
Fire
Water
Democritus
• Greek philosophers
recognised that there were
distinct elements each with
unique properties.
Democritis in the 5th
Centaury BC proposed that
all matter was made of very
small, indivisible, solid
particles. Other prominent
philosophers such of his day
rejected his ideas.
• The concept of atoms lay
forgotten for 1200 years
John Dalton
• John Dalton (1766 –
1844) reintroduced the
Greek concept of very
small, indivisible
particles.
Joseph Thomson
• Joseph Thomson (1856 –
1940) discovered that the
atom contained both
negative and positive
charges. He proposed that
the atom was a ball of
positive charge with electrons
(negative charges) evenly
distributed through it, making
it overall electrically neutral.
(The ‘plum pudding’ model).
Ernest Rutherford
• Ernest Rutherford (1871 - 1937)
set out to prove Thomson’s
model, but ended up discovering
that atoms were composed of a
small, dense nucleus (99.95% of
the atom’s mass) containing
positively charged protons and
surrounded by electrons which
orbited the nucleus. Most of the
atom was empty space between
the electrons and the small,
dense nucleus.
The Challenge of Proof
• In order to prove that
Thomson’s model of the atom
was correct Rutherford had to
make observations and take
measurements of atoms that
where too small for him to see,
touch, taste, smell or hear.
• Rather like identifying a
mystery object in a cardboard
box that you weren’t allowed
to open
Rutherford’s experiment was really a very clever idea
Rutherford’s Gold Foil Experiment
• He fired helium nuclei (small +ve
charged particles), at high speed,
at a very thin layer of gold (about
400 atoms thick) and recorded
where they hit a fluorescent
screen
• He expected that most would
pass straight through the “plum
pudding” and that a few would
be deflected slightly if they
happened to pass close to one of
the “raisins” (negative charges electrons)
• The positive dough, he thought
would have little or no effect
because the charge was so
diffuse (widely spread out)
Rutherford’s Gold Foil Experiment
Http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
Rutherford’s Findings
• From Thomson’s model
Rutherford expected most of
the alpha particles to pass
straight through the gold
atoms
• His results showed some alpha
particles were deflected
through a large angle and some
even bounced back towards
the source; “Like howitzer
shells bouncing off of tissue
paper!”
Howitzer
Rutherford’s Nuclear Atomic Model
From his observations and measurements
Rutherford developed a nuclear model of the atom
with;
• A small positively charged nucleus at the centre
of the atom (further experiments showed that the
nucleus contained protons)
• Most (99.95%) of the mass of the atom in the
nucleus
• Atom is mainly empty space
• Electrons that are very light (small mass) and orbit
the nucleus
Additions to Rutherford’s Model
Later
• By 1919 Niels Bohr a Danish proposed
electron orbitals or shells, a concept that lead
to quantum theory
• In 1932 James Chadwick identified the neutral
particles in the nucleus with the same mass as
protons
Rutherford’s Nuclear Model of the Atom
• Even this diagram is far from scale. From Rutherford’s
experiment a scale model would be more like a marble at
the centre of a rugby field as the nucleus and the atoms
as grains of coarse sea salt orbiting the top of the
grandstand (there’s a lot of empty space in an atom)
Your Understanding
• You need to have a good understanding of
both Thompson’s and Rutherford’s models of
the atom and
• You also be able to describe Rutherford’s gold
foil experiment, what he observed, what he
concluded and why.
• It is certainly well worth practising answers
to questions including diagrams and
explanations