The Evolution of the Atom

Democritus

460 BCE

Democritus was a Greek philosopher who believed that atoms were indivisible, tiny solid sphere. He believed atoms couldn't be destroyed. Because atoms are much too small for the naked eye to observe, his research and ideas were ignored for around 2000 years.

John Dalton

1808

Dalton came up with what is known as the atomic theory, saying that Democritus was right all along, despite never being believed, and that all matter was made up of tiny, individual particles. He came to this conclusion after examining the work of other scientists, as well as by doing his own experiments.

J.J. Thompson

1897

Thompson, by carrying out an experiment using a cathode ray and bending it using electrical and magnetic fields. This caused Thompson to believe that atoms consisted of negatively charged particles called electrons. He knew that atoms were electrically neutral, so he believed atoms were also made up of a positive sphere, which cancelled out negative electrons.

Ernest Rutherford

1911

Ernest Rutherford discovered protons. He did this when firing positive alpha particles at an extremely thin sheet of gold foil. If Thompson had the correct model of the atom, alpha particles would have gone straight through the foil, however, some particles were deflected slightly. This experiment lead Rutherford to believe that a small, dense mass is located at the centre of an atom, which he named the nucleus. He believed it was positively charged to repel the positive alpha particles.

Niels Bohr

1913

Bohr discovered that electrons orbit the nucleus in orbits of different energy and are located further and further away from the nucleus. These were known as shells.Together with Chadwick, his Bohr-Chadwick model is used today.

James Chadwick

1932

James Chadwick, a student of Rutherford, believed that the nucleus contained a neutral subatomic particle, otherwise the protons in the nucleus would repel each other and rip the atom apart.