The ancient Greeks believed there was only 4 elements (water, air, fire and earth). They also predicted that no matter how many times you break something it can always get smaller.
Greek philosopher Democritus suggested that everything was made up of tiny unbreakable particles. Democritus was the first to use the word atomos meaning invisible which is where the word atom come from.
English chemist John Dalton theorised that all matter is made up of hard, invisible spheres.
British scientist Joesph John Thomson discovered the electron in 1897. Knowing that the electron was negatively charged he knew there must be a positively charge in the atom to keep it balanced.
In 1904 he proposed the Plum Pudding Model which showed a positively charged ball with electrons embedded in it.
Hungarian scientist Phillip Lenard described atoms as mostly empty space with fast moving neutrally charged particles called 'dynamides'. These dynamaides were said to be made up of a positively charged particle and a negatively charged particle stuck together.
In 1909, New Zealand scientist Ernest Rutherford preformed an experiment in which he fired a beam of Alpha particles (positively charged) at gold foil. He found that although most went through some bounced back.
In 1911, he created the nuclear model which had a small nucleus with a large space filled with electrons.
Niels Bohr, a danish scientist, proposed that electrons can only go in certain pathways around the nucleus known as orbits. It is called the Planetary Model because the electrons orbit around the nucleus like planets around the sun. English scientist James Chadwick modified Bohr's model after his discovery of neutrons. He found that the nucleus wasn't just a positively charged ball but a cluster of positively charged protons and neutrally charged neutrons.
The Electron Cloud Model is the most recent atomic model. The model suggests that there is no specific location in which that electrons can be found. Instead the electrons create a cloud around the nucleus.