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400 BCE
Democritus
Greek philosopher Democritus believed that everything consisted of extremely small particles that could not be divided. He explained this by snapping a stick and saying there must be a point where it can no longer be halved. These were called atoms from the Greek word atomos, which meant “uncut” or “indivisible.” He believed that there was different types of atoms with specific sets of properties. -
John Dalton
John Dalton made multiple points for his theory: All elements are composed of atoms; All atoms of the same element have the same mass, and atoms of different elements have different masses and that Compounds contain atoms of more that one element -
JJ Thomson
JJ Thomson's experiments provided evidence that atoms are made of even smaller particles that previous scientists had thought. In his model of the atom, the negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter. -
Ernest Rutherford
Ernest Rutherford developed the model of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun. The model showed all of an atom’s positive charge is concentrated in its nucleus. -
Niels Bohr
Niels Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. In the Bohr Model the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun. -
Erwin Schrödinger
Erwin Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This is known as the quantum mechanical model of the atom. The quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. Werner Heisenberg contribution to the atomic theory was that he calculated the behaviour of electrons, and subatomic particles that also make up an atom.
