The History of the Atomic Model

Ancient Greeks

Time Line

1803

John Dalton

Dalton noted that oxygen and carbon combined to make two compounds. Each had its own weight ratio of oxygen to carbon but for the same amount of carbon, one had twice the amount of oxygen than the other. His atomic theory stated that elements consisted of tiny particles called atoms. He believed that the reason the element is pure is because all atoms of an element were identical and had the same mass. Dalton also stated that the reason elements were different from each other because the atoms of each element were different and had different masses. Dalton’s model was that the atoms were tiny, invisible and indestructible particles and that each one had a certain mass, size and chemical behaviour. That was determined by what kind of element they were.

In 1897 he discovered the electron and the fact that it had a negative charge. Thompson knew that there must be a source of positive charge in the atom due to the atom because neutral charge. This lead to him discovering the plum pudding model in 1904. In this model, an atom is thought of as a round ball of positive charge with negatively charged electrons embedded in it. Like plums in a plum pudding, hence the name plum pudding model.

A Danish scientist by the name of Niels Bohr, modified the nuclear model and proposed that electrons can only travel along certain pathways around the nucleus, this is called orbits. As a outcome, this model is sometimes called the planetary model. This model explained why different elements produce different coloured light when the light is heated. This observation is due to the electrons moving from higher to lower orbits and emitting coloured light in the process.

Rutherford, a New Zealand scientist, performed an experiment where he fired a beam of positively charged alpha particles at gold foil. He found that most of the alpha particles went through the foil. But a small number were deflected. This lead to the development of the nuclear model of the atom. In which most of the mass is thought to be contained in a small positive nucleus, which is surrounded by a large space. This space is thought to be occupied by negative electrons.

The ancient Greeks believed that all matter was made up of four basic elements. Earth, Fire, Air and Water. This is what made up the base of the continuum model. Which predicts that regardless the number of times you halve a piece of matter, it can always be broken into even smaller pieces.

1904

Joseph John Thompson

1911

Ernest Rutherford

1913

1932

Niels Bohr

James Chadwick

Early BC

The History of the Atomic Modelby Hannah ' Georgia

An English scientist discovered the neutron, showing that the nucleus was not just a mass of positive charge but it was a cluster of positively charged protons and charge- neutral neutrons. This model is called the planetary model with neutrons.