Covalent Bonding: Single, Double, and Triple Bonds

A covalent bond is a chemical bond formed when two atoms share one or more pairs of electrons; a single bond shares one pair, a double bond shares two pairs and a triple bond shares three pairs. In brief: in H₂ two atoms share one pair, in O₂ they share two pairs, and in N₂ they share three pairs; as the number of shared pairs increases, the bond becomes shorter and stronger.

Covalent bonding is the way nonmetal atoms combine to form molecules, from water and carbon dioxide to proteins and DNA. This article explains how electron pairs are shared, describes single, double and triple bonds with examples, compares bond lengths and energies in a table, introduces sigma and pi bonds, works through examples of counting electrons and bonds, describes the properties of covalent substances and gives the history of the theory.

How do atoms share electrons?

When two nonmetal atoms come together, neither can easily lose electrons. They can both reach a stable electron arrangement by sharing electrons. A shared pair is attracted to both nuclei at once, which holds the atoms together. In a hydrogen molecule, each atom contributes one electron, and the shared pair gives each nucleus access to two electrons, the duet of helium. Electron pairs that belong to one atom only are called lone pairs, and pairs shared between two atoms are called bonding pairs.

What are single, double and triple bonds?

• Single bond: one shared pair (2 electrons). Examples are H–H, the bonds in H₂O (H–O–H), CH₄ and NH₃, and the C–C bond in ethane. A single bond is shown as one line.
• Double bond: two shared pairs (4 electrons), as in O=O, CO₂ (O=C=O) and ethene (H₂C=CH₂). It is shown as two lines.
• Triple bond: three shared pairs (6 electrons), as in N≡N, ethyne (HC≡CH), carbon monoxide (C≡O) and hydrogen cyanide (HC≡N). It is shown as three lines.

Atoms form multiple bonds to complete their octets when single bonds are not enough. Nitrogen has 5 valence electrons and needs 3 more, so two nitrogen atoms share three pairs. Carbon, nitrogen, oxygen and sulfur commonly form multiple bonds. Hydrogen and the halogens form only single bonds.

Bond lengths and bond energies

Bond length is the distance between the two nuclei, and bond energy is the energy required to break one mole of bonds in the gas phase. Values are averages and vary slightly with the molecule.

The very strong triple bond in N₂ explains why nitrogen gas is so unreactive and why breaking it, as in the manufacture of fertilizer, needs high temperatures and a catalyst.

What are sigma and pi bonds?

In valence bond theory, a sigma (σ) bond is formed by the overlap of orbitals along the line joining the nuclei, and it can exist alone. A pi (π) bond is formed by the sideways overlap of p orbitals above and below that line. A single bond is one σ bond. A double bond is one σ and one π bond, and a triple bond is one σ and two π bonds. Pi bonds are weaker than sigma bonds and are more exposed, so they are the usual site of chemical reactions in alkenes and alkynes. A double bond prevents rotation about the bond, which gives rise to cis and trans isomers.

What are the properties of covalent substances?

• Low melting and boiling points, because molecules are held together by weak intermolecular forces. Methane boils at −162 °C.
• Poor electrical conduction, since there are no free ions or electrons (exceptions: graphite, and acids that ionize in water).
• Often soft solids, liquids or gases at room temperature.
• Solubility depends on polarity: polar molecules dissolve in water, and nonpolar ones in nonpolar solvents.

Worked examples

1. Count electrons for CO₂. Valence electrons: 4 + 2 × 6 = 16. Two double bonds use 8 electrons, and the remaining 8 form two lone pairs on each oxygen atom. Each atom has an octet.
2. Number of σ and π bonds in ethene (C₂H₄). Four C–H σ bonds, one C–C σ bond and one C–C π bond: 5 σ and 1 π.
3. Number of σ and π bonds in HCN. H–C is one σ bond, and C≡N is one σ and two π bonds: 2 σ and 2 π.
4. Predict the multiple bond. N₂ has 10 valence electrons. A single bond would leave each N with only 6 electrons. A triple bond with one lone pair on each atom gives each nitrogen 8 electrons.
5. Energy to break a mole of N₂. 945 kJ for 28.02 g, or 945 ÷ 28.02 = 33.7 kJ per gram.
6. Compare lengths. The C=C bond (134 pm) is shorter than the C–C bond (154 pm) by 20 pm, which is a decrease of 20 ÷ 154 × 100% = 13%. The C≡C bond (120 pm) is shorter by 34 pm, or 22%.

What are network covalent solids?

In some substances, atoms are linked by covalent bonds throughout the whole crystal, forming a giant lattice. Examples are diamond, where each carbon atom bonds with four others, and silicon dioxide (quartz). These substances are extremely hard and have very high melting points, such as 3,550 °C for diamond (under pressure), because many strong covalent bonds must be broken. Graphite is a variation with layers of carbon atoms, in which delocalized electrons allow electrical conduction.

How did the theory of covalent bonding develop?

In 1916, G. N. Lewis proposed that a chemical bond is a shared pair of electrons. In 1927, Walter Heitler and Fritz London gave the first quantum-mechanical treatment of the hydrogen molecule, which explained the covalent bond as a result of the overlap of atomic orbitals and the exchange of electrons. Linus Pauling developed valence bond theory and hybridization in the early 1930s, and Robert Mulliken and Friedrich Hund developed molecular orbital theory at about the same time.

Which mistakes are common?

Mistake 1: Counting shared electrons as belonging to one atom only

In an octet count, a shared pair counts for both atoms.

Mistake 2: Thinking that a double bond is twice as strong as a single bond

The C=C bond (614 kJ/mol) is less than twice the C–C bond (346 kJ/mol), since the π bond is weaker than the σ bond.

Mistake 3: Forgetting the lone pairs

Lone pairs affect the shape of molecules and their reactivity, so they must be shown.

Mistake 4: Assuming that all covalent substances are gases or liquids

Diamond and quartz are covalent solids with very high melting points.