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400 BCE
Democritus
He said that atoms are indivisible and indestructible. He was way ahead of his time for this stuff and set the stage for all future scientists after him and all the way through to today. -
John Dalton, the Atomic Theory
He discovered that everything is made up of atoms. Atoms of the same element are extremely similar to each other and atoms can be rearranged but never can change -
Dmitri Mendeleev
Mendeleev is known for his work on the Periodic Law and creation of the first Periodic table. In 1869 he created the first Periodic Table. The Periodic Law states that when elements are arranged according to their atomic number, elements with similar properties will appear at regular intervals. -
Eugen Goldstein
He discovered that protons have an equal and opposite charge as an electron. He used cathodes rays and at the same time discovered canal rays. -
J.J Thomson
In 1897, J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. He made a plum pudding model to show this but Thomson's model of the atom did explain some of the electrical properties of the atom due to the electrons, but failed to recognize the positive charges in the atom as particles.
Per http://www.abcte.org/files/previews/chemistry/s1_p5.html -
Ernest Rutherford
Created the Rutherford gold foil Experiment, a series of experiments by which scientists discovered that every atom has a nucleus where all of its positive charge and most of its mass is concentrated. -
Robert Millikan
He conducted the oil drop experiment to determine the charge of an electron. They suspended tiny charged droplets of oil between two metal electrodes by balancing downward gravitational force with upward drag and electric forces. In 1910 Robert Millikan succeeded in precisely determining the magnitude of the electron's charge. -
Niels Bohr
He proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. ... When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr's theory could explain why atoms emitted light in fixed wavelengths. -
James Chadwick
Up until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. ... This particle became known as the neutron.
