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Posted by on Aug 3, 2011 in General Chemistry, Quantum Mechanics Research | 0 comments

Early models of the Atom

Models for the atom

Once atoms were found, it became evident that atoms were composed of protons, neutrons and electrons.  The first model, due to J.J. Thomson, was the plum pudding model.  This suggested that the atom was a positively charged cloud with electrons floating in it.  This model is incorrect.

J.J. Thomson, plum pudding model of atom

All models in science have to be consistent with experiment.  While at McGill University in Montreal, Ernest Rutherford did a series of scattering experiments that proved the Plum pudding model was incorrect.

Rutherford fired alpha particles at a thin sheet of metal foil.  Alpha particles are He nuclei or He 2+ atoms.  If the Thompson model was correct, all the alpha particles would pass right through the atom.  In fact most did.

Rutherford fired alpha particles at a thin sheet of metal foil

However, a small number of alpha particles bounced right back.  This could only happen if they hit a very dense part of the atom. Rutherford was surprised at this and said:

“It is as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

The only way to explain this result is that a very dense nucleus exists in the center of the atom and this is surrounded by electrons.  This dense nucleus was occasionally hit by the alpha particles and deflected.  When the nucleus was not hit, the alpha particle went straight through the atom.  The model of the atom with a dense nucleus at the center and surrounded by electrons is called the Rutherford model of the atom.

Rutherford model of the atom

The Bohr model states that electrons exist in various orbitals and the energy corresponds to the energy level diagram on the left.  Electrons of higher energy occupy Bohr orbitals of greater radii.

Bohr Model of Atom

The statistical interpretation of quantum mechanics is one of the most profound aspects of science. It was developed in Copenhagen between 1925 and 1930 and was not fully accepted by Einstein who said

“God does not roll dice.”

But today most scientists agree Heisenberg was right. There is an inherent fundamental error to measurement.

Quantum Atom





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