In hydrogen gas (H₂), the electrons are shared equally by the two hydrogen atoms. In many bonds, however, the electrons are not shared equally by the two atoms. A notable example is provided by the bonds in a water molecule (H₂O), which consists of two hydrogen atoms each covalently bound to a single oxygen atom (Fig. 2.7).

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In a molecule of water, the electrons are more likely to be located near the oxygen atom. The unequal sharing of electrons results from a difference in the ability of the atoms to attract electrons, a property known as electronegativity. Electronegativity tends to increase across a row in the periodic table; as the number of protons across a row increases, electrons are held more tightly to the nucleus. Therefore, oxygen is more electronegative than hydrogen and attracts electrons more readily than does hydrogen. In a molecule of water, oxygen has a slight negative charge, while the two hydrogen atoms have a slight positive charge (Fig. 2.7). When electrons are shared unequally between the two atoms, the resulting interaction is described as a polar covalent bond.

A covalent bond between atoms that have the same, or nearly the same, electronegativity is a nonpolar covalent bond, which means that the atoms share the bonding electron pair almost equally. Nonpolar covalent bonds include those in gaseous hydrogen (H₂) and oxygen (O₂), as well as carbon–carbon (C–C) and carbon–hydrogen (C–H) bonds. Molecules held together by nonpolar covalent bonds are important in cells because they do not mix well with water.