In simple terms
A covalent bond is what holds two atoms together when they share electrons. Molecules are built from them.
Sharing suits atoms whose outer electron shells are incomplete. Each atom contributes, and both end up more stable than they were apart.
How it works
IUPAC describes a covalent bond as a region of relatively high electron density between nuclei, arising at least partly from the sharing of electrons, which produces an attractive force and a characteristic distance between the nuclei.
How many bonds an atom forms depends on its outer shell. Hydrogen forms one, oxygen up to two, nitrogen a maximum of three, and carbon up to four in a tetrahedral arrangement. That fourfold capacity is why carbon builds the long chains and rings of organic chemistry.
Strength matters too. A typical covalent bond runs about 50 to 100 kilocalories per mole, roughly 210 to 420 kilojoules per mole, around a hundred times the energy of random thermal motion. That is why molecules survive at room temperature.
A double bond is shorter and stronger than a single bond, and it blocks rotation, so that part of the molecule is stiffer.
When the two atoms pull on the shared electrons unequally, the bond is polar. The uneven charge creates a permanent dipole, and those dipoles drive much of the chemistry of water and of living cells.
Why it matters
Covalent bonds are why matter is stable without being permanent. They are strong enough to survive heat and motion, yet enzymes and reactions can still rearrange them.
DNA, proteins, plastics, fuels and medicines are all held together this way.
Where you’ll see it
- Structural formulas, where each line stands for a shared pair
- Descriptions of DNA and protein structure
- Polymer and materials science
- Explanations of why water behaves so oddly
Example
In a water molecule, one oxygen atom forms a covalent bond with each of two hydrogen atoms. Oxygen pulls the shared electrons harder, so both bonds are polar.
Often confused with
A covalent bond is not a hydrogen bond. Hydrogen bonds are noncovalent and much weaker, about 1 to 7 kilocalories per mole, which is why they form and break constantly inside cells.
Key facts
- IUPAC defines a covalent bond as a region of relatively high electron density between nuclei, arising at least partly from sharing of electrons, giving an attractive force and a characteristic internuclear distance.1
- Typical covalent bonds run about 50 to 100 kilocalories per mole, roughly a hundred times thermal energies, while noncovalent bonds are about 1 to 7 kilocalories per mole.2
- Hydrogen forms one covalent bond, oxygen up to two, nitrogen a maximum of three, and carbon up to four in a tetrahedral arrangement.2
- Double bonds are shorter and stronger than single bonds and prevent rotation, making that part of a molecule more rigid.2
- When bonded atoms attract the shared electrons unequally, the bond is polar and carries a permanent dipole.2
Related concepts
Quick checkHow many covalent bonds can a carbon atom form, and why does it matter?Show answer
Up to four, arranged tetrahedrally. That capacity lets carbon build the long chains and rings of organic chemistry.
Sources
- International Union of Pure and Applied Chemistry. Compendium of Chemical Terminology (Gold Book), entry C01384: covalent bond. Undated (accessed 16 September 2026)
- Alberts B, Johnson A, Lewis J and others, Molecular Biology of the Cell, 4th edition, Garland Science, via NCBI Bookshelf. The Chemical Components of a Cell. 2002 (accessed 16 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 16, 2026Researched and drafted with AI assistanceReport an issue