Aristotle believed that matter was made of combinations of earth, fire, water, and air. That led Alchemists to experiment with matter to try to turn common metals into Gold.
Aristotle's 4-element theory lasted for about 2000 years! However, it was not a scientific theory because it could not be tested.
Here are some significant scientists who studied the atom:
Democritus: A greek philosopher who believed that atoms were indivisible spheres that could not be seen with the naked eye.
Lavoisier: Discovered conservation of mass, and the law of definite proportions.
Proust: Discovered that if a compound is to be broken down, that the products would exist in the same ratio as the original compound- proving Lavoisier's laws.
Dalton: Still believed that atoms were solid indestructible spheres, but states that every element has a different type of atom. Stated that atoms of one type of element can combine with the atoms of another to form chemical compounds, and that atoms cannot be created nor destroyed.
JJ Thomson: Created the "raisin bun" model: solid positive spheres, with negative particles embedded within. This was the first atomic theory to have positive and negative charges! He used the cathode tube experiment to demonstrate the existence of electrons.
Rutherford: Showed the atoms have a dense positive center (nucleus), with electrons outside of it. He discovered this with the gold leaf experiment, which resulted in the planetary model of the atom. It explains the reason why electrons spin around the nucleus. It also suggested that atoms were mostly empty space.
Niels Bohr: Studied gaseous samples of atoms, which glowed when an electric current passed through them. Niels Bohr proposed that electrons surround the nucleus in different energy levels, and can jump from one to another.
Of course, now we know that the modern atom is made of three kinds of particles called SUBATOMIC particles, which include:
The atom on the left is what scientists thought they looked like- solid indivisible spheres. This changed when JJ Thomson discovered the existence of protons and electrons!
On the right is the modern atom.