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492 BCE
Empedocles
He proposed one of the first theories that attempted to describe the things around us. Empedocles argued that all matter was composed of four elements: fire, air, water, and earth. -
430 BCE
Democritus
He theorized that all material bodies are made up of indivisibly small “atoms.” -
John Dalton
He propose a modern theory of the atom based on the following assumptions, matter is made up of atoms that are indivisible and indestructible. -
J.J. Thomson
He discovered the electron and then went on to propose a model for the structure of the atom. -
Ernest Rutherford
Ernest Rutherford found that the atom is mostly empty space, with nearly all of its mass concentrated in a tiny central nucleus. -
Niels Bohr
Niels Bohr proposed a model of the atom in which the electron was able to occupy only certain orbits around the nucleus. This atomic model was the first to use quantum theory, in that the electrons were limited to specific orbits around the nucleus. -
Louis DeBroglie
He introduced the idea that particles, such as electrons, could be described not only as particles but also as waves. -
Werner Heisenberg
He contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly. -
Erwin Schrödinger
He showed that the quantization of the hydrogen atom's energy levels that appeared in Niels Bohr's atomic model could be calculated from the Schrödinger equation, which describes how the wave function of a quantum mechanical system. -
James Chadwick
James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced.