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The History of the Atomic Model: Thomson and the Plum Pudding

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J.J Thomson contributed massively to the model of the atom and the modern day theory. His work involved the use of cathode ray tubes and identifying a particle lighter than the atom itself, the electron.

Atoms for a long time were thought to be the smallest part of matter and the work of Avogadro into the volumes of gases in 1811 and Robert Brown on Brownian motion further solidified the work of Dalton and the atomists that all matter was made of small indivisible particles, unique in nature as elements. However, in 1897 the English physicist J.J Thomson discovered the electron as a particle through his work with cathode ray tubes. Thomson used a sealed glass container with two electrodes either side to apply a voltage through a vacuum which creates a glow which strikes the glass tube and can be deflected by electric fields.

Thomson concluded that these rays were not light but instead made of tiny particles. He measured the mass of these and discovered they are 1800 times smaller than that of the element hydrogen. This led him to conclude that these rays were particles and a smaller piece of matter than the atom itself. He further concluded that these small particles were building blocks of larger particles and that they must be negatively charged. He theorised that to balance out this negative charge they must be surrounded by a positive charge. This led him to hypothesise the plum pudding model of the atom, where a sea of negative particles (electrons they would be named later) were surrounded by a cloud of positive charge to balance them out.


Terms in section
Robert Brown

Robert Brown was a Scottish scientist who observed small particles in solution moving randomly. His theory came to be known as Brownian motion

Brownian Motion

Brownian motion is a term used to describe the movement of small particles in random directions and was used as evidence to show that atoms existed due to their ability to collide with each other.

Avogadro

Amadeo Avogado was an Italian scientist who discovered the relationship between gases and the number of particles in a volume of a gas. He also discovered the mole as a quantity to describe the number of particles in a given quantity that could be exchanged between a solid, liquid and gas.

J.J Thomson

J Thomson was an English physicist who is credited with discovering the electron and theorising the first model of the atom to use sub atomic particles. He proposed the plum pudding model.

Electron

The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus

Cathode ray tube

The Cathode ray tube is a device which uses electrons being fired through a vacuum onto a screen. J.J Thomson used this to predict the mass of an electron and its existence as a particle smaller than the atom.

Electrode

An electrode is a conductive part of an electrical circuit that is usually in contact with a non-metallic substance.

Vacuum

A vacuum exists when all particles have been removed from a container creating a space with no particles in.

Plum Pudding Model

The plum pudding model was suggested as the first atomic model by J.J Thomson where he suggested that the atom was a sea of positive charge that surrounded small negative electrons

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The History of the Atomic Model: Rutherford and Bohr

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