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The Discovery of the Nuclear Atom: Rutherford (1911)
We mow know that an atom consists of a nucleus surrounded by an electron cloud1. However
this understanding of the structure of atoms largely arises from a famous ‘gold foil experiment’
that was carried out in 1911 by three Scientists - Rutherford, Geiger and Marsden. Before this
scientists tended to think of atoms to consist of negatively-charged electrons within a posively
charged medium, sometimes referred to as the ‘plum pudding’ model. For example the following
diagrams illustrate our current understanding of the atomic structure of Lithium and the one
suggested by the ‘plum pudding’ model.
Atomic structure of 6Li.
‘Plum pudding’ model for 6Li
The experiment involved monitoring how a beam of α particles from a radioactive source were
scattered when fired at very thin sheets of gold. (Alpha particles are , are positively charged and
are much smaller than an atom, but much larger than an electron and are the result of
radioactive decay2.) In the experiment a circular flourescent screen was used to detect the α
particles; when an α particles impacts on a fluorescent screen then a flash or scintillation occurs.
The configuration of the experiment is shown in the following diagram.
fluorescent screen
α particles
α emitter
gold leaf
Atomic Structure and the Periodic Table
2 Radioactive Decay
1
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The plum pudding model would suggest that the α particles would pass through the metal
either with a very small deflection given the attractive force of the electrons. However, a very
small proportion (about one in 10,000) of the scintillations were found to occur on the same
side of the gold sheet as the αsource, as if they bounced off the gold sheet. Rutherford suggested
a new structure – termed the nuclear atom - for the atom based on this experiment. The nuclear
atom consists mainly of empty space and the vast majority of its mass is concentrated into a
positively charged and very small central core which was named the nucleus. The nucleus is
approximately 10,000 times smaller than the atom.
The nuclear atom explains the results of Rutherford’s experiment, as illustrated in the following
diagram. Most alpha particles pass straight through the gold foil. However, alpha particles with
a trajectory clost to a hard positive nucleus are strongly deflected or rebound.
Some are deflected
Some rebound
Most pass straight through
Rutherford’s nuclear atomic model explains the results of the gold leaf experiment.