Rutherford replaced the plum pudding model with the nuclear model

• In 1804, John Dalton agreed with Democritus that matter was made up of tiny spheres that couldn’t be broken up, but he reckoned that each element was made up of a different type of atom

• Nearly 100 years later, J.J.Thompson discovered particles called electrons that could be removed from atoms. So Dalton’s theory wasn’t quite right. Thomson suggested atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding-the plum pudding model

• However, in 1909, scientists in Rutherford’s lab tried firing a beam of alpha particles at thin gold foil-this was the alpha scattering experiment. From the plum pudding model, they expected the particles to pass straight through the gold sheet, or only be slightly deflected. But although most of the particles did go straight through the sheet, some were deflected more than expected, and a few were deflected back the way they had come-something the plum pudding model couldn’t explain

• Because a few alpha particles were deflected back, the scientists realised that most of the mass of the atom must be concentrated at the centre in a tiny nucleus. This nucleus must also have a positive charge, since it repelled the positive alpha particles

• They also realised that because nearly all the alpha particles passed straight through, most of an atom is just empty space. This was the first nuclear model of the atom.

Which developed into the current model of the atom

• The nuclear model that resulted from the alpha particle scattering experiment was a positively charged nucleus surrounded by a cloud of negative electrons

• Niels Bohr said that electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculations agreed with experimental data

• Evidence from further experiments changed the model to have a nucleus made up of a group of particles which all had the same positive charge that added up to the overall charge of the nucleus

• About 20 years after the idea of a nucleus was accepted, in 1932, James Chadwick proved the existence of the neutron, which explained the imbalance between the atomic and mass numbers

Current model of the atom

• It contains protons and neutrons, which gives it an overall positive charge

• The rest of mostly empty space, negative electrons more around the outside of the nucleus really fast

• Radius of atom is 1x10(-10)

• Number of protons = number of electrons

• If they gain energy by absorbing EM radiation they move to a higher energy level