The development of the model of the atom

New experimental evidence may lead to a scientific model being changed or replaced.

[[Solid spheres - John Dalton[[

Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided, and they are separated from each other by empty space.

}}Plum pudding model - JJ Thompson}}

The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.

{{Nuclear model{{

• Scientists fired positively charged particles into a thin sheet of gold. Their idea was if the positive charge was generally spread out, the particles should pass the sheet of gold. The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. Some particles were deflected to the side. This told the scientists that the centre of the atom must be positively charged and that repelled the alpha particles.

Most of the alpha particles went straight through gold foil.

Therefore atoms are mainly empty space.

Some alpha particles also bounced straight back which means that the mass of the atom must be concentrated in the centre. Today, the central part is called the nucleus. This nuclear model replaced the plum pudding model.

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances and they were held in shells. This was an important idea because orbiting of electrons prevents the atom from collapsing. The theoretical calculations of Bohr agreed with experimental observations.

{{Protons and neutrons{{

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.