-
450 BCE
Empedocles
Figured out air is a substance -
Period: 450 BCE to
Atomic Timeline
-
430 BCE
Democritus
Atoms have their own size and shape. Atoms are "uncuttable", they are the smallest particles of matter. -
350 BCE
Aristotle
Did not agree with Democritus (shadowed over him). Believed everything was a combination of the 4 elements (Earth, Wind, Fire, Water). Greatly slowed the theory. -
John Dalton
Each element has its own unique property. All matter are made up of tiny particles called atoms. All atoms of a given element are alike, but are different from the atoms of any other elements. Compounds are formed when atoms of different element combine in fixed proportions. A chemical reaction involves the rearrangement of atoms, not a change in atoms themselves. Atoms can't be created, destroyed, or divided, only changed. -
J.J. Thomson
Discovered that atoms are made up of smaller negatively charged particles called electrons and positively charged matter. He discovered this by doing experiments with "cathode ray tubes" Created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in. He referred to it as the plum pudding model. -
Albert Einstein
Proved that Brownian Motion of Particles (1827 theory of Robert Brown, Botanist) do exist with his mathematical calculations of watching pollen jitter in water.
Through statistics, he helped others advance and explain their models with real data. -
Ernest Rutherford
He decided to shoot a beam of tiny, positively charged particles at a thin sheet of gold foil. He thought that most of the particles would pass right through the gold atoms, which they did. They deflected sideways as well. What surprised him was that some particles bounced right back. Based on the results of his experiment, he proposed a new model of the atom, called the nuclear model. In it, the center of the atom is a tiny, dense positively charged area called the nucleus. -
Niels Bohr
Electrons move with a constant speed in fixed, planet like orbits around the nucleus. Electrons move in paths at certain distances around the nucleus. Electrons can move from a path on one level to a path on another level. -
G.N. Lewis
Figured out valence electrons. Covalent bonding of elements. Placed dots (no more that 8) around the symbol of each element to represent the outermost electrons, the Lewis-Dot Diagram. -
Werner Heisenberg (with help from Niels Bohr)
Uncertainty principle: electrons don’t travel in orbits, just certain regions. -
Erwin Schrodinger
Began explaining the wave theory. Electrons move in a wave. Electrons are found in electron clouds, not paths. -
James Chadwick
Discovered Neutrons in the nucleus of the Atom (neutral particles). Neutrons have a mass nearly equal to the mass of a proton. Unit of measurement for subatomic particles is the atomic mass unit (amu). Assistant to Rutherford. Nobel Prize in 1935. Advanced the work of nuclear fission and the model of the atom.
