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Democritus was the first person to ever use the word atom and he briefly described the "Atom" and how it was what matter was made up.

330 b.c

The first model of the atom was developed by JJ Thomson, who thought that atoms were composed purely of negatively charged electrons. This model was known as the 'plum pudding' model.

1904

This theory was disproved by Ernest Rutherford and the gold foil experiment, where Rutherford shot alpha particles at gold foil, and noticed that some went through and some bounced back, implying the existence of a positive nucleus.

1911

Niels Bohr proposed a model of the atom where the electrons were contained within quantized shells that orbited the nucleus. This was because it was impossible for the cloud of negative electrons proposed by Rutherford to exist, as the negative electrons would be drawn to the positive nucleus, and the atom would collapse in on itself.

1913

However the model used today is closest to the Bohr model of the atom, using the quantized shells to contain the electrons.