Isotopes are variations of atoms of the same element. The element is defined as the number of protons (atomic number), which is invariable, but the number of neutrons, the mass number or atomic weight, can vary.

From Greek [isos = ‘the same’, topos = ‘location’], isotopes are listed on the Periodic Table as the one element (same location, whole number atomic number), and the relative proportions of the varying atomic weights is usually expressed as a single average.

Some isotopes are radioactive. They can be used as fissile material in nuclear reactors to generate electricity because they can be induced to decay at a rate far greater than the natural decay rate by bombardment with neutrons.

Isotopes are also used as radiation sources in applications such as medicine and research, and they make useful markers for radiometric dating of substances. An example is carbon-14.

Isotopes should not be confused with allotropes, which are different shapes or bond combinations of the same element. An example of allotropes are graphite and diamond.