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Period: 400 to
Documented Reseach
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470
Democritus, Greece.
The idea that everything is composed of atoms was brought about by a Greek philosopher named Democritus, but another, more influential philosopher did not believe this, and dismissed the idea. Democritus said that atoms are physically but not geometrically indivisible, that they are indestructible, between them there is empty space, they have always been and always will be in motion, there is an infinite number of atoms and infinite kinds of atoms which all differ in shape and size. -
Isaac Newton, England.
Newton defined how forces can affect small particles across a distance. Newton helped us to understand the forces that can affect small particles. -
John Dalton, England.
Dalton said that elements consisted of tiny particles called atoms, and that the reason an element is pure is because all the atoms of an element are identical, and that they have the same mass. His theory also stated that atoms of each element were different from one another; in particular, they had different masses which helped lead to his conclusion that each atom has its own characteristic weight. He said that compounds were atoms of different elements put together. This helped classify atom -
George Johnstone Stoney, Ireland.
He proposed the term “electron”, to describe the essential unit of electrical charge. Stoney introduced the idea of electrons, helping us to figure out the composition of atoms. -
J. J. Thomson
He discovered through his studies that atoms are also comprised of other things- protons and electrons. He also determined the charge to mass ratio of an electron (=1.759 x 10 8 coulombs/gram). -
Max Planck, Germany.
Planck discovered quantum theory, by studying the nature of light and other radiation. He hypothesised at very small scales, that matter was discrete rather than continuous. This means that we know that atoms behave differently to the human-scale world that we know and perceive. Used the idea of quanta (discrete units of energy) to explain hot glowing matter. -
Hantaro Nagaoka, Japan.
Nagaoka developed an early, incorrect "planetary model" of the atom which was based around an analogy to the explanation of the stability of Saturn’s rings. It had a massive nucleus, with electrons revolving around the nucleus, bound by electrostatic forces. However, this analogy was wrong although the electrons revolving around the nucleus was correct, and was abandoned in 1908. -
Robert Millikan, USA.
Millikan measured the charge on an electron with his oil-drop apparatus. He also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. -
Ernest Rutherford, New Zealand.
Rutherford formulated a model of the atom, which had: a very small positively charged nucleus, orbited by electrons. He also speculated on the existence of ‘neutrons’, He concluded that the nucleus was very small, very dense and assumed that electrons were outside of it. He is also credited with discovering the proton. -
Henry G.J. Moseley, England.
He wrote that the atomic number of an element is equal to the number of protons in a nucleus. This information was used to reorganize the periodic table according to atomic number instead of atomic mass. -
Niels Bohr, England.
He proposed that each electron shell going out would have more electrons than the previous ones and that the outer shells determine the atom’s chemical properties. Bohr contributed with the idea of electronic configuration. -
James Chadwick, England.
Chadwick discovered a third type of subatomic particle and named it the ‘neutron’. They help stabilize the protons in the atom’s nucleus, preventing the protons from repelling each other. Neutrons are always in the nucleus of atoms, and are about the same size as protons however, they have no electrical charge.