Understanding the basics of covalent bonding is important in environmental science. When two atoms join together in a covalent bond, they form a molecule that shares electrons. Unlike in the ionic bond, neither of the atoms in a covalent bond loses or gains an electron; instead, both atoms use a pair of shared electrons.

The simplest covalent bonds form between atoms of the same element. For example, two oxygen atoms join together in a covalent bond to form a molecule of O2 gas (see the figure for a visual of this molecule).


To help predict which atoms are likely to form covalent bonds, scientists use the octet rule. This rule states that atoms are most stable (and, thus, nonreactive) when their first electron shell has two electrons and any outer shells have eight electrons.

Atoms that form covalent bonds are slightly unstable because they don’t have enough electrons in their outer electron shell. When one slightly unstable atom combines with another slightly unstable atom and they share their unstable electrons with each other, they both become more stable. As a result, the molecule the two atoms created is unlikely to react with other atoms around it; in other words, it becomes less chemically reactive.

Understanding covalent bonding is particularly important in environmental science when you study organic matter, or matter that contains carbon. The element carbon has four electrons in its outer shell and can form covalent bonds with up to four other atoms at one time. By forming so many covalent bonds, carbon molecules can build large, complex shapes.