DemocritusDemocritus - 442 BC
Postulated the theory of atoms and void; the universe is made
up of empty space and a nearly infinite number or invisible
particles differing in arrangement, form, and position. He
stated that all all matter is composed of indivisible atoms
John DaltonEnglish chemist who proposed the Atomic Theory which
stated that all matter was composed of small indivisible
particles called atoms. These atoms possess unique
characteristics and weight for different elembets.
He also stated that three types of atoms exist: simple
(elements), compound (simple molecules), and complex
J.J Thomson1998 First to measure the electron. Developed the
plum-pudding model of the atom. Describes the atom as a
slightly positive sphere with small electrons inside.
Inferred a small, dense, positively-charged nucleus as the
result of the alpha particle scattering gold foil experiment.
A Danish physicist who stated that electrons could only
orbit the nucleus in succesively larger orbits around the
nucleus. The outer orbits could hold more electrons. The
electrons in the outermost shell determine the properties of
the atom. He also inferred that when an electron moves from
an outer orbit to an inner orbit it emits a quantum of energy,
a photon, in the form of light.
Developed wave mechanics which describe the behavior
of quantum systems for bosons.