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An atom is the basic unit of matter. Different types of atoms have different chemical properties and are called elements. Throughout the course of history human understanding of the atom has changed over time. As new evidence is collected, the theory of the atomic model changes.
1808 - John Dalton
Proposed the idea that atoms are like hard balls that can't be broken into smaller pieces.
1897 - J.J. Thomson
Thomson found that atoms contained negatively charged particles and invented the plum pudding model of the atom. In this model, electrons are surrounded by positively charged "pudding".
1911 - Ernest Rutherford
Rutherford's model stated that an atom is mostly empty space around a small nucleus at the center. The electrons in Rutherford's model orbit randomly around the nucleus.
1913 - Neils Bohr
Discovered that electrons can only have certain amounts of energy. Electrons exist at different energy levels described as orbits in Bohr's model of the atom.
1932 - James Chadwick
Discovered the neutron as a part of the atom. Neutrons are neutrally charged particles found in the nucleus of an atom.