Elements: Chemical Symbols and Isotopes

An element is a pure substance that contains only one type of atom. Currently, 118 elements have been discovered, either occurring in nature or synthesized in a laboratory.

Elements vary widely in their abundance. The most abundant elements in the Earth’s crust are oxygen, silicon, aluminum, iron, and calcium. In comparison, the Earth’s atmosphere is mostly made up of nitrogen and oxygen.

On the periodic table, each square contains, at a minimum, an atomic number and a chemical symbol.

A chemical symbol is a one- or two-letter abbreviation given to an element. The first letter is capitalized and the second, if necessary, is lowercase to avoid confusion with compounds that contain two elements. Often, this symbol is the first letter or letters of the element’s English name. However, some symbols stem from Greek, Latin or German roots.

Additional details can be added to a chemical symbol. The atomic number, which is also the number of protons, is noted in the left subscript position. This number always is the same for a given element and is sometimes omitted, as it does not change.

The mass number — the number of protons plus neutrons — is indicated in the left superscript position. This number defines the specific isotope of an element, such as carbon-12 or carbon-13.

An isotope is a variation of an element where the number of neutrons in the nucleus varies but the number of protons — also the atomic number — is the same. This results in different atomic masses but does not affect the charge of the atom. For instance, elemental hydrogen has three naturally occurring isotopes: The most abundant form contains no neutrons; the second is deuterium, containing one neutron; and the third is tritium, which contains two neutrons.

While the standard hydrogen atom and deuterium are stable under natural conditions, other isotopes, such as tritium, decay into other substances by the emission of energy. These are known as radioactive isotopes.