Atoms of the same element with the same number of protons but a different number of neutrons (different mass numbers) are known as isotopes.

The mass numbers or isotopes of an element are denoted as preceding superscripts. For example, the stable isotopes of the element oxygen are denoted 18O, 17O, and 16O. Oxygen has an atomic number of 8 (eight protons). The nucleus of 16O thus contains eight protons and eight neutrons.

Because elements may have several stable isotopes, the average mass number of an element is the atomic weight and is commonly not an integer.

Some isotopes may emit neutrons, protons, and electrons, and attain a more stable atomic configuration (lower level of potential energy); these are radioactive isotopes or radioisotopes. Radioactive decay (carbon-14 decaying to eventually become nitrogen-14) describes the energy loss that occurs when an unstable atom’s nucleus releases radiation.

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