-
400 BCE
Democritus
The theory of Democritus held that everything is composed of "atoms," that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms. -
John Dalton
Dalton's atomic theory proposed that all matter was composed of atoms, indivisible, and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass. -
Dmitri Mendeleev
In 1869 Dmitri Mendeleev created the first Periodic Table. Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. -
Robert Millikan
Robert Millikan was a physicist who discovered the elementary charge of an electron using the oil-drop experiment. His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory. -
Eugene Goldstein
He was a German Physicist that discovered that protons have an equal and opposite charge as an electron. He used cathode rays and at the same time discovered canal rays. He assumed that these new rays had equal and opposite charge of the electron. -
Ernest Rutherford
Ernest Rutherford found that the atom is mostly empty space, with nearly all of its mass concentrated in a tiny central nucleus. The nucleus is positively charged and surrounded at a great distance by the negatively charged electrons. -
Niels Bohr
Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well-defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. -
J.J Thomson
J.J. Thomson is credited with the discovery of the electron, the negatively charged particle in the atom. He is known for the Thomson atomic theory. In 1904, Thomson proposed a model of the atom as a sphere of positive matter with electrons positioned based on electrostatic forces. -
James Chadwick
In 1932, James Chadwick bombarded beryllium atoms with alpha particles. Unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron.
