Chemical elements make up all matter, whether it contains life or not. These chemical elements are fundamental in the sense that they are what they are and cannot be converted into another element. Carbon, hydrogen, and oxygen are the three elements that are most prevalent in living things. The majority of living matter is composed of these three elements, nitrogen, phosphorus, and a few additional components. A chemical element's atom is its lone constituent.

A chemical element is a species of atoms, including the pure substance made entirely of that species, that have a specific number of protons in their nuclei. Chemical elements, in contrast to chemical compounds, cannot be reduced by any chemical process into simpler molecules. The distinguishing characteristic of an element is its atomic number, which is the number of protons in the nucleus; all atoms with the same atomic number are atoms of the same element.

Chemical elements make up almost all of the universe's baryonic stuff. Atoms are rearranged into new compounds linked together by chemical bonds when various elements undergo chemical reactions. A small number of relatively pure native element minerals, including silver and gold, are found uncombined. On Earth, nearly every other naturally occurring element exists as a combination or mixture. Although it does contain other substances like carbon dioxide and water, the main constituents of air are the elements nitrogen, oxygen, and argon.

The periodic table, which powerfully and elegantly arranges the elements by increasing atomic number into rows in which the columns have recurrent physical and chemical properties, is frequently used to summarize the properties of the chemical elements.