Atomic Models

Atomic models have their beginnings in the early atomic theory. The law of conservation of mass, the law of definite proportions, and the law of multiple proportions, provided models for the behavior of chemical reactions, but the laws could not be explained. In 1808, John Dalton (1766–1844) proposed his atomic theory, which served as an explanation for these phenomena. His theory consists of five postulates. The first postulate states that all matter is composed of atoms. Second, the atoms of a given element are all exactly the same in structure and properties. Third, atoms cannot be further divided, created, nor destroyed. Fourth, atoms of different elements combine to form compounds. Lastly, chemical reactions involve the combination, separation, and rearrangement of atoms. These five postulates not only explained the laws of conservation of mass, definite proportions, and multiple proportions, but also served as the basis for the study and development of various atomic models.

Not all of Dalton's atomic theory has stood the test of time. The evolution of the atomic model has led to the discovery that atoms are divisible into smaller particles, and that not all atoms of a given element are exactly the same. In the late 1800s, scientists discovered that atoms are composed of three subatomic particles, called protons, neutrons, and electrons. The protons and neutrons are located in a central region of the atom called the nucleus, and the electrons occupy the space surrounding the nucleus. The number and arrangement of each of these particles within the atom determine the chemical properties of the element. Atoms of the same element can differ in the number of neutrons (called isotopes) and also in the number of electrons (called ions). The number of protons is what determines the chemical identity of the element. The discovery of these subatomic particles, along with Dalton's atomic theory, set in motion the development of several atomic models. Advances in modern physics allowed scientists to find a large number of subatomic particles, including the quarks that are the fundamental subatomic particles that form protons and neutrons.