Sharing electrons
A covalent bond forms between two non-metal atoms when they share one or more pairs of electrons. Each shared pair counts toward the octet (or duplet) of both atoms, so neither atom has to gain or lose electrons completely.
Single, double and triple bonds
- Single bond — one shared pair, e.g. H–H in H2 or Cl–Cl in Cl2.
- Double bond — two shared pairs, e.g. O=O in O2 and each C=O in CO2.
- Triple bond — three shared pairs, e.g. N≡N in N2.
Common covalent molecules include water H2O, ammonia NH3 and methane CH4.
Worked example / 例题
How many covalent bonds does carbon form in methane? Carbon (2.4) needs 4 more electrons; hydrogen (1) needs 1. Carbon shares one electron with each of four hydrogen atoms, forming four single C–H bonds. Formula: CH4. Carbon now has 8 valence electrons and each H has a duplet.
Key idea / 要点
The number of covalent bonds an atom forms equals the number of electrons it needs to complete its octet: O forms 2, N forms 3, C forms 4, and H or halogens form 1.
Why covalent substances differ
Covalent bonds within a molecule are strong, but the forces between molecules are weak. This is why many covalent (molecular) substances have low melting and boiling points and do not conduct electricity.
Remember / 记住
- Covalent bonds form between non-metals only.
- A double bond shares two pairs; a triple bond shares three pairs.
- Shared electrons are counted by both bonded atoms.