Glossary
covalent bond
A covalent bond is a shared pair of electrons between two atoms, the usual bond between nonmetals and the bond that holds molecules together.
A covalent bond is a pair of electrons shared between two atoms, and it is how nonmetals bond: neither atom is willing to give an electron away, so they share instead. Sharing one pair is a single bond, two pairs a double bond, three a triple; the more pairs shared, the shorter and stronger the bond. Water, carbon dioxide, glucose and every protein are held together this way, which is why the chemistry of living things is the chemistry of covalent bonds. The shared electrons usually sit closer to the more electronegative atom, and when the pull is very uneven the bond is polar — the reason water dissolves salts and why one end of a molecule can be slightly negative and the other slightly positive. A molecule is a group of atoms joined by covalent bonds; the bonds inside it are strong, while the forces between separate molecules are much weaker.
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Related terms
- ionic bond — An ionic bond is the attraction between two oppositely charged ions, formed when one atom gives electrons to another — a metal and a nonmetal, usually.
- hydrogen bond — A hydrogen bond is a weak attraction between a hydrogen already bonded to oxygen, nitrogen or fluorine and a nearby lone pair on another molecule, or on another part of the same molecule.
- valence electron — A valence electron sits in an atom's outermost occupied shell, and the count there decides which bonds the atom forms and what charge its ion carries.
- acid — An acid is a substance that gives up hydrogen ions in water, which lowers pH below 7 and lets it neutralize a base.
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)