The History of The Atomic Model

Atoms, derived from the Greek word ‘Atomos’, meaning indivisible, make up everything in the world. They are the smallest part of elements that can exist, but what exactly is the structure of an atom like on the inside?

Protons, neutrons and electrons, which are subatomic particles, make up atoms. Each proton has a relative charge of +1 (positive), each electron, a relative charge of -1 (negative) and each neutron, which has a neutral charge. In atoms, the number of protons will always equal the number of neutrons, which is why atoms have no charge (the charges cancel out). Atoms have a positive nucleus made up of protons and neutrons, with electrons orbiting around them in electron shells. The electrons stay in their shells because there is a force of attraction between them and the positive nucleus.  It took us a very long time to be able to find out all of this. In the past, the atomic model was completely different to the one accepted today. Here is the history of the atomic model.

Source: Pixabay. This is the atomic model.

Democritus was a Greek philosopher around 400BC who first thought of what the structure of an atom was like. He believed that all atoms were invisible, had nothing inside and cannot be destroyed. Afterwards, the next point of progress happened much later, around the 1800s, by a man called John Dalton. He thought that all atoms were solid spheres and that atoms of the same element were of identical sizes. Atoms of different elements, therefore, would be of different sizes.

Then around 100 years later, J.J Thomson measured the masses and charges of atoms which led him to discover electrons. As atoms have no charge and electrons have a negative charge, Thomson figured out that positive charges must also exist to balance out all charges. This resulted in the plum pudding model.

In the model, atoms were made of a ball of positive charge with electrons stuck in it (just like a plum pudding).

10 years later, the theory was disproven by Ernest Rutherford through an experiment he did which was known as ‘the alpha scattering experiment’. In the experiment, he fired positive alpha particles at a thin gold sheet. The result was that some particles were repelled by the centre of the atom, proving that atoms have a positively charged centre. Rutherford’s new model had atoms with a positive nucleus in the centre  surrounded by electrons. Most of the atom was just empty space.

However, there is a problem with this model, if electrons were able to move in a cloud without any restrictions, then they would automatically be attracted to the positive centre, which would make the atom collapse. To counter this issue,  Niels Bohr proposed that electrons have fixed areas which they are allowed to orbit around the nucleus in, and these were called electron shells. Bohr suggested that these shells were at fixed distances from the nucleus.

Bohr’s model was different to Rutherford’s model; the electrons could only orbit around the nucleus in electron shells.

Finally, the last two scientists who contributed to the atomic model, was once again Rutherford who through further experiments, found out that the nucleus was made of smaller particles. Rutherford discovered the proton and the second scientist, James Chadwick, discovered the neutron. This atomic model is similar to the one accepted today.

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