Atomic Models

Dalton's & Thomson's Models

19th century: John Dalton - solid spheres, different spheres made up different elements
1897: J.J. Thomson - concluded from experiments atom must contain even smaller negatively charged particles - electrons
'Solid sphere' idea of atomic structure had to be changed
New model: 'plum pudding model'

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1909 - Ernest Rutherford: gold foil experiment
fired alpha particles at v thin sheet of gold
expected from plum pudding model: most alpha particles deflected slightly by positive 'pudding'
most of alpha particles passed straight through gold atoms
v small number deflected backwards
came up with nuclear model of atom
tiny, positively charged nucleus at centre, surrounded by 'cloud' of negative electrons
most of atom empty space

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scientists realised that electrons in 'cloud' around nucleus would quickly spiral down into nucleus: atom collapse
Niels Bohr proposed new model with 4 basic principles:
- electrons only exist in fixed orbits & not anywhere in between
- each shell - fixed energy
- when electron moved between shells - electromagentic radiation emitted/absorbed
- energy of shells is fixed so radiation has fixed frequency
frequencies of radiation emitted & absorbed by atoms was already known from experiments
Bohr model fitted these observations

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scientists later discovered not all electrons in a shell had same energy
meant Bohr model not quite right
refined model fit observations even better
refined model is not perfect but widely used to describe atoms because it is simple & explains many observations from experiments
most accurate model is based on quantum mechanics
explains some observations that can not be accounred for by Bohr model, lot harder to get head round & visualise
scientists use whichever model is most relevant to whatever they're investigating

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