-
400 BCE
Democritus
Greek philosopher named the smallest piece of matter atomos, meaning not to cut. -
John Dalton
Dalton's theory stated that all matter was made up of atoms. The atoms could not be created nor destroyed. All atoms of certain elements are identical. Every substance is made up of elements combined in certain ways. -
J.J. Thomson
Atoms are made up of even smaller particles. Thomson thought the atom was a positive sphere with negatively charged electrons mixed through it. -
Ernest Rutherford
Conducted an experiment to study the parts of the atom. He suggested that the atoms have a nucleus, a small dense center with a positive charge surrounded by moving electrons. He discovered the protons. -
Niels Bohr
Bohr agreed that the atom has a positive nucleus surrounded by electrons. In his model, the electrons move around the nucleus in circular paths. Each path is a certain distance from the nucleus. -
James Chadwick
Chadwick discovered that the nucleus contains uncharged particles called the neutrons. The electrons do not move in circular paths but in an area called the electron cloud.
