Non-metals form covalent bonds (molecular bonds) by sharing an electron-pair between them. For most atoms, the sharing of an electron-pair in a covalent bond allows them to obtain a stable electron configuration, following the octet rule.

If the two bonding atoms have different electronegativities it results in the electron pair not being shared equally between the two atoms. If the difference in electronegativity is greater than 0.4 on the Pauling scale the bond is said to be polar. If the difference is greater than 1.7 the bond is considered to be mainly ionic. It is not a sharp divide though, but a matter of the bond being predominantly ionic when above 1.7.

On the left is a chlorine atom with 7 valence electrons. Chlorine is electronegative as shown by the lowercase delta negative symbol. On the right is a hydrogen atom with one valence electron. Hydrogen is electropositive as shown by the lowercase delta positive symbol. The chlorine atom and hydrogen atom are close together so their electron orbitals overlap. Hydrogen and chlorine share 2 electrons between each other to form a covalent bond.

Figure 1 Polar covalent bond between chlorine and hydrogen. The distribution of electron density in the HCl molecule is uneven. Symbols δ+ and δ– indicate the polarity of the H–Cl bond.

The covalent bond between the atoms can be a σ (sigma) bond or a π (pi) bond, depending on the hybridization of the bonding orbitals.