Isotopes are atoms whose nuclei contain the same number of protons but a different number of neutrons. The term
is derived from Greek (meaning equal places) and indicates that isotopes occupy the same position in the periodic table.
It is convenient to denote isotopes in the form
E, where the superscript
denotes the mass number (i.e., sum of the number of protons and neutrons in the nucleus) and the subscript
denotes the atomic number of an element, E. For example,
C is the isotope of carbon which has six protons and six neutrons in its nucleus. The atomic weight of each naturally occurring element is the average of the weights contributed by its various isotopes.