What is a covalent bond?
Quick answer
A covalent bond is a chemical bond in which two atoms share one or more pairs of electrons. Sharing lets each atom fill its outer electron shell, making both more stable. Covalent bonds usually form between nonmetal atoms, and they hold together molecules such as water (H₂O), oxygen (O₂) and carbon dioxide (CO₂).
Why atoms share
Atoms are most stable with a full outer shell of electrons, usually eight (or two for hydrogen). Nonmetal atoms tend to need a few more electrons. Instead of one atom taking electrons from another, they share them, so both can count the shared pair as their own. Electrons are explained in which part of an atom has a negative charge.
Single, double and triple bonds
| Bond | Shared pairs | Example |
|---|---|---|
| Single | 1 pair | H–H (hydrogen gas); C–H in methane |
| Double | 2 pairs | O=O (oxygen gas); C=O in carbon dioxide |
| Triple | 3 pairs | N≡N (nitrogen gas) |
Triple bonds are very strong, which is why the nitrogen in our air is so unreactive; see the most abundant gas in the atmosphere.
Equal and unequal sharing
| Type | What happens | Example |
|---|---|---|
| Nonpolar covalent | Electrons shared equally | O₂, H₂ |
| Polar covalent | One atom pulls electrons harder, creating slight charges | Water: oxygen pulls electrons away from hydrogen |
Water’s polarity makes it great at dissolving things and gives it many of its unusual properties.
Covalent vs ionic vs metallic
| Bond | How it works | Typical atoms | Example |
|---|---|---|---|
| Covalent | Electrons shared | Nonmetal + nonmetal | Water, sugar |
| Ionic | Electrons transferred; opposite ions attract | Metal + nonmetal | Table salt; see the formula for table salt |
| Metallic | A “sea” of shared electrons | Metal atoms | Copper, iron |
Where you find covalent bonds
Almost all the molecules of life, including DNA, proteins and sugars, are held together by covalent bonds, mostly involving carbon; see the element with atomic number 6. Diamond is a giant network of covalent bonds, which makes it so hard.
Who described it?
American chemist Gilbert N. Lewis proposed the idea of shared electron pairs in 1916. He also invented Lewis dot diagrams, still used in classrooms to show how atoms bond.
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