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The accepted model of the atom has changed throughout history. The model of the atom that
is currently accepted fits all the observations and evidence we have so far, so we assume its
true until someone shows that it is incomplete or wrong. In the past, completely different models
were accepted, because they fitted the evidence at the time.
Some ancient Greeks thought that all matter was made from indivisible particles. At the start of
the 19th century John Dalton described atoms as solid spheres, and said that different types of
sphere made up the different elements. But as scientists did more experiments, our currently
accepted models began to emerge, with modifications or refinements being made to take
account of new evidence.
Experimental evidence showed that atoms weren't solid spheres. In 1897 JJ Thompson did
a whole series of experiments and concluded that atoms weren't solid and indivisible. His
measurements of charge and mass showed that an atom must contain smaller, negatively
charged particles. He called these particles `corpuscles'- we call them electrons. The `solid
sphere' idea of atomic structure had to be changed. The new model was known as the plum
pudding model- a positively charged sphere with negative electrons embedded in it.
Rutherford showed that the plum pudding model was wrong however. In 1909, Ernest
Rutherford and his students Hans Geiger and Ernest Marsden conducted the famous Gold Foil
experiment. They fired alpha particles, which are positively charged, at an extremely thin sheets
og gold. From the plum pudding model, they were expecting most of the alpha particles to be
deflected very slightly by the positive pudding that made up most of an atom. In fact, most of the
alpha particles passed straight through the gold atoms, and a very small number were deflected
backwards. This showed that the plum pudding model couldn't be right. So Rutherford can up
with a model that could explain this new evidence- the nuclear model of the atom:
A few alpha particles Most of the alpha
are deflected very pass straight through the
strongly by the empty space
There is a tiny, positively charged nucleus at the center of the atom, where most of the atom's
mass is concentrated. The nucleus is surrounded by a `cloud' of negative electrons and most of
the atom is empty space.
Rutherford's model was modified several times. It seemed pretty convincing but the scientists
of the day, continued with their experiments, wanting to be sure of the truth. Henry Moseley
discovered the charge of the nucleus increase from one element to another in units of one.
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This led Rutherford to investigate the nucleus further. He finally discovered that it contained
positively charged particles that he called protons. The charges of the nuclei of different atoms
could then be explained- the atoms of different elements have a different number of protons
in their nucleus. there was still one problem with the model though- the nuclei of atoms were
heavier than tney would be if they contained just protons.…read more