Covalent

Covalent bonding is a type of chemical bond in which electron pairs are shared between atoms, typically nonmetals with similar electronegativities. By sharing electrons, each atom can attain a stable electron configuration, often an octet, without complete electron transfer. The shared electrons reside in overlapping atomic orbitals and constitute a covalent bond. Bond strength and length depend on bond order: single bonds are weaker and longer than double bonds, which are weaker and longer than triple bonds.

Most covalent bonds occur in molecules or molecular compounds, such as H2, H2O, CH4, and O2. In these substances, covalent bonds hold the atoms together within discrete units; intermolecular forces such as van der Waals interactions and, in some cases, hydrogen bonding determine physical properties like boiling points and solubility.

Polarity arises when electronegativity differences between bonded atoms produce partial charges, yielding polar covalent bonds (for example, HCl). If electronegativities are similar, bonds are nonpolar covalent, as in H2, N2, or O2.

In solids, covalent bonding can also form extensive networks, or covalent networks, where atoms are linked by a continuous array of covalent bonds. Diamond and silicon carbide are classic examples, giving high hardness and high melting points. By contrast, molecular covalent compounds consist of discrete molecules bound by weaker intermolecular forces.

Covalent bonding contrasts with ionic bonding, which results from complete electron transfer and electrostatic attraction, and with metallic bonding, where electrons are delocalized. Covalent bonds can be cleaved by chemical reactions, requiring energy input to break the shared electron pairs.