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460 BCE
Democritus
Democritus discovered that everything is made out of invisible atoms that are so small they are indivisible. He also said that atoms entirely fill the space that they are in. -
John Dalton
John Dalton created a atomic theory. His four main ideas of atomic theory are that all matter is made of atoms, all atoms of a specific element have a fixed mass, compounds have atoms from different elements and the different elements in compounds are combined in a specific way. -
Marie Curie
Marie Curie discovered radium and polonium while investigating radioactive substances. -
J.J. Thompson
J.J. Thompson discovered the electron and produced the “plum pudding” model of the atom which includes electrons scattered around a spherical cloud of positive charge. -
E. Goldstein
Goldstein discovered protons. He thought that since atoms had negatively charged particle, they had to also have positively charged particles to stay neutral. -
Robert Millikan
Robert Millikan found out the electric charge of an electron. -
Ernest Rutherford
Ernest Rutherford used the gold-foil experiment to discover that all of a atom’s mass is in small, positively charged balls at the center of the atom called the nucleus. -
Niels Bohr
Niels Bohr stated that electrons move around the nucleus in orbits with fixed sizes and energy. -
Quantum Mechanical Model
The Quantum Mechanical Model is based on the theory that matter has properties associated with waves and that it is impossible to know both the exact position and momentum of an electron at the same time, which is known as the Uncertainty Principle. This model has electron cloudsor orbitals orbiting the nucleus. The clouds are volumes of space where it is likely there is an electron. -
James Chadwick
James Chadwick discovered the neutron. He said that there were neutrally charged particles called neutrons in the nucleus of an atom.
