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400 BCE
Democritus discovers the atom
In 400 B.C., Greek philosopher Democritus suggested the existence of the atom. He believed that atoms uniform and indestructible. Democritus invented the Solid Sphere Model to illustrate his atomic idea. His theory was not well supported because it did not follow the scientific method. -
Dalton Defines the Atom
In 1803, Dalton came up with his own atomic theory. This theory stated:
1. Matter is composed of tiny indivisible particles called atoms
2. All atoms of the one element are identical but are different from the atoms of other elements
3. Atoms of different elements can combine in ratios to form compounds -
JJ Thomson
JJ Thomson believed that atoms consist of a large sphere of uniform positive charge embedded with smaller negatively charged particles. He made this discovery using a cathode ray tube. He invented the “Plum Pudding Model” to detail his ideas. -
Rutherford’s Experiment
Ernest Rutherford performed the “Gold Foil Experiment” where he fired alpha particle at a sheet of gold foil. With this experiment he concluded that the atom’s nucleus has a positive charge. Rutherford believed that the atom’s mass is concentrated in the nucleus. -
The Bohr Model
Bohr believed that electrons move around the nucleus in orbits of fixed sizes and energies and that the electrons further away from the nucleus are easier to remove. He thought if the atom absorbed energy, the electron jumped to a level further from the nucleus, and if it radiated energy, it fell to a level closer to the nucleus. -
Werner Heisenberg
Heisenberg applied a mathematical system to atomic physics, called matrix mechanics. He realized measuring an electron’s properties by hitting it with gamma rays would alter the electron’s behavior. Heisenberg became the director of the German atom bomb project. -
Schrodinger’s Cloud Model
Edwin Schrödinger’s idea was that electrons move in “clouds” instead of orbitals. He thought electrons move in set paths around the nucleus, but in waves, making it impossible to know the exact location of electrons. This is accepted as the most accurate atomic model today. -
James Chadwick
Chadwick believed the nucleus of an atom contains neutrons and protons which makes up most of an atom’s mass. Chadwick’s discovery of the neutron made the development of the atomic bomb possible.