Aristolte• Four “elements”: earth, fire, water, air
• Believed in reasoning instead of scientific experiments
• Dismissed Democritus’ album as “worthless” The Aristotle atomic theory may not be a concept that the great man is really remembered for. He disagreed with the prevailing theories of the time. He was not able to see how atoms could stay in perpetual motion in a void. He developed a theory that was based on the four elements Earth,Fire,Water,Air.
Period: 300 to
Democritus of Adbera400B.C.
• Atomos: indivisible
• First to identify possibility of an atom
• Smallest piece of matter
The theory of Democritus and Leucippus held that everything is composed of "atoms", which are physically, but not matmatically, indvisible; that between atoms lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.
John dalton• Father of modern atomic theory
• Includes elements are composed of atoms
• Five parts to theory
Also includes element’s atoms are identical in mass
Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom.
J.J. Thompson• Described nature of cathode rays
• Plum pudding model of the atom
• Discovered electron
Thompson discovered that electrons were smaller particles of an atom and were negatively charged.
Earnest Rutherford• Gold foil experiment
• Atom has small positive nucleus, remainder is empty space
• Predicted existence of neutrons
Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.
Niels Bohr• Planetary model of the atom
• Electrons travel in specified energy levels
• Spectrum lines produced when electrons move
Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus