Democritus was a Greek philosopher from 400 B.C. and he suggested the first atomic theory. He helped lead on other scientist to get to where we are today. On thing he also proposed was explaining all things are "composed of minute, invisible, indestructible particles of pure matter which move about eternally in infinite empty."
Dalton was a British scientist from Eaglesfeild, England. He was also a schoolmaster and scientist. Dalton made inferences in the 19th century that exhibited how atoms bond together in definite proportions. He was considered the first to discover the Atomic Theroy.
Sir William Crookes was born in London, England and helped with the atomic theory. When preforming a spectroscopic examination he saw a green line of residue left in the manufacture of sulphuric acid. Sir William Crookes had just discovered a new element called thallium.
Dmitri Mendeleev was the reason we now have the periodic table. He was born in Tobolsk, Siberia. Mendeleev created his own version of the periodic table and it was sorted out by atomic mass. He also won the Copley Medal and the Demidov Prize.
J.J. Thomson was a scientist who was born in a small suburb, Cheetham Hill, in side of Manchester, England. He was born in the year 1856 but later on was a big part in science, he ended up contributing to more than just the atomic theory. Thomson discovered that atoms can be cut. He found this out while studying rays traveling between charged metal plates in a vacuum tube. He discovered that the rays were consisted of negatively charged particles. He had just discovered electrons. He discovered all of this on a trip from America. Since electrons are so small, he believed that they could only be from inside of the atom. Since Thomson's discovery of the electron, John Dalton's theory that atoms are indivisible had to be changed.
Albert Einstein was born in Ulm, Germany. In 1905 Einstein made a ground breaking advancement. He pointed the way towards getting test done for the reality of atoms and of the validity of the laws of thermodynamics.
Ernest Rutherford was born in Nelson, New Zealand. Rutherford was also widely known for his gold foil experiment. In his experiment he used a machine that would shoot alpha particles at a thin sheet of gold to prove that atoms are mostly empty space.
Henry was born in Weymouth, Dorset, on the southwestern coast of England. Moseley was the first to clear and scientifically justify in 1913 the atomic number studying X-ray spectra of chemical elements.
Niels Bohr, who was born in Copenhagen, Denmark, stated that he thought that electrons orbit around the nucleus of an atom in a planet like fashion. Witch later on was concluded true. Also in 1922 he received the Nobel Prize in Physics.