Atomic theory describes what atoms are made of and how they behave. Some of the earliest philosophers and scientists had long suspected that unseen, tiny particles existed. As chemists learned more about elements, the first well-developed atomic theory was formed by the English chemist John Dalton. Dalton theorized that all matter is made of atoms, which are indivisible and indestructible. He also stated that atoms of a given element have identical physical properties and that compounds are formed by combinations of atoms. A chemical reaction, however, can change the compounds by rearranging the atoms. Over time, other scientists, such as the English physicists J.J. Thomson and Ernest Rutherford, along with the Danish physicist Niels Bohr, discovered the subatomic particles that make up the atom and refined the model of atomic structure. English physicist James Chadwick discovered the neutron, the existence of which revealed isotopes, which are different forms of the same element that have different numbers of neutrons. Modern atomic theory has thus refined Dalton's original theory to account for subatomic particles and isotopes.
At A Glance
Atomic theory is based on the work of John Dalton, whose experiments led to the understanding of atomic properties and behavior.
John Dalton proposed that elements are composed of identical, indivisible atoms, compounds are formed by combining atoms, and chemical reactions change compounds by rearranging the atoms.
- J.J. Thomson developed the plum pudding model of the atom, in which negatively charged electrons float in a positively charged material.
- Ernest Rutherford proposed the nuclear model of the atom, in which electrons orbit a positively charged nucleus that contains almost all the mass of the atom.
Niels Bohr proposed the Bohr model of the atom, in which electrons orbit the nucleus in shells that lie at a specific distance from the nucleus, and electrons are not found outside these shells.
- The experiments of Robert Millikan, Erwin Schrödinger, and James Chadwick led to the discovery of subatomic particles and isotopes, which shaped modern-day atomic theory.
Atoms can be described in terms of their size, atomic number, atomic weight, and ionic charge.