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400 BCE
Democratus
He contributed to the theory of atomism, propounded by his teacher Leucippus, by suggesting that all matter consisted of tiny, indivisible particles. Democritus made many very important discoveries in the span of his lifetime but the greatest was that of the atom. If it weren't for Democritus, there would be no modern atomic theory and there would still be questions left to answer that were answered hundreds of years ago. Instead of experiments he used his analogies to prove that he’s right. -
John Dalton
In 1803 he revealed the concept of Dalton’s Law of Partial Pressures. Also in the 1800s, he was the first scientist to explain the behavior of atoms in terms of the measurement of weight. Dalton's experiments on gases led to his discovery that the total pressure of a mixture of gases amounted to the sum of the partial pressures that each individual gas exerted while occupying the same space.
In 1803, Dalton also created the first chart of atomic weights. -
JJ Thomson
In 1897 Thomson discovered the electron and then went on to propose a model for the structure of the atom. His work also led to the invention of the mass spectrograph. A couple of other things that Thomson discovered was the subatomic particle and the isotope. One experiment that he did was In 1897. Thomson set out to prove that the cathode rays produced from the cathode were actually a stream of negatively charged particles called electrons.
He had referred to Maxwell’s theory as well. -
Ernest Rutherford
In 1899, Rutherford had discovered alpha and beta rays from uranium. Now, today we know they are not rays, they are particles; alpha is a nucleus of helium and beta is an electron. In 1911, he was the first to discover that atoms have a small charged nucleus surrounded by largely empty space, and are circled by tiny electrons (model). An experiment he was apart of was the good foil experiment. When he shot a beam of alpha particles at a sheet of gold foil, a few of the particles were deflected. -
Niels Bohr
In 1913, Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element. He had created the Bohr model. In 1913, the model depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus. Bohr's greatest contribution to modern physics was the atomic model His experiment was with his model to depict the atom.
