Atoms and Isotopes

In the atomic theory proposed by John Dalton in 1805, all atoms of a given element were assumed to be identical. Eventually it was realized that atoms of a given element are not necessarily identical; an element can exist in several isotopic forms that differ in atomic mass.

Every atom has a positively charged nucleus and one or more electrons that form a charge cloud surrounding the nucleus. The nucleus contains over 99.9% of the total mass of the atom. Every nucleus may be described as being made up of two different kinds of particles, protons and neutrons, collectively called nucleons.

Protons and neutrons have nearly the same mass, but only the proton is charged, so that the total charge of a nucleus is equal to the number of protons times the charge of one proton. The magnitude of the proton charge is equal to that of the electron so that a neutral atom has an equal number of protons and electrons.

The atoms of all isotopes of an element have the same number of protons, the atomic number, Z. The nuclei of different isotopes differ, however, in the number of neutrons and therefore in the total number of nucleons per nucleus. The total number of nucleons is A, the mass number. Atoms of different isotopic forms of an element, uclides, are distinguished by using the mass number as a left superscript on the symbol of the element, e.g., ¹⁵N refers to the isotope of N with mass number 15.