Covalent Bonding: Sharing Electrons

1Why this matters

The methane dissolved deep in Lake Kivu, the water you drink, the oxygen you breathe and the carbon dioxide you breathe out are all made of molecules: small groups of non-metal atoms joined together. Ammonia, the starting point for most of the nitrogen fertilisers used on Rwandan farms, is another.

Two non-metal atoms cannot bond by transferring electrons, because both need to gain electrons. Instead, they share them. By the end of this lesson you will be able to draw dot-and-cross diagrams for simple molecules, recognise single, double and triple bonds, and explain the properties of simple molecular substances.

2The covalent bond

A covalent bond is a pair of electrons shared between two atoms. Each atom provides one electron of the pair, and both atoms count the shared pair as part of their own outer shell. The shared pair is attracted to both nuclei, and that attraction holds the atoms together. A molecule is a group of two or more atoms held together by covalent bonds.

Worked example 1: hydrogen and chlorine molecules

A hydrogen atom has 1 electron and needs 1 more to reach the helium duplet. Two hydrogen atoms each put their electron into a shared pair. Each atom now "owns" 2 electrons. We write the molecule as H–H, where the line stands for one shared pair: a single bond.

A chlorine atom (2,8,7) has 7 outer electrons and needs 1 more. Two chlorine atoms share one pair, so each has 8 outer electrons: Cl–Cl. The three pairs on each chlorine that are not shared are called lone pairs.

Hydrogen chloride, HCl, forms in the same way: one shared pair between H and Cl, giving H a duplet and Cl an octet.

Worked example 2: water, H₂O

Oxygen (2,6) has 6 outer electrons and needs 2 more. Each of two hydrogen atoms shares one pair with the oxygen. In the dot-and-cross diagram, the oxygen's outer shell holds 8 electrons: 2 shared pairs (each one • from oxygen and one × from a hydrogen) and 2 lone pairs. The structure is H–O–H.

Worked example 3: methane, CH₄

Carbon (2,4) has 4 outer electrons and needs 4 more. It shares one pair with each of four hydrogen atoms, forming four single bonds. Carbon now has 8 outer electrons and each hydrogen has 2. There are no lone pairs on the carbon.

Worked example 4: ammonia, NH₃

Nitrogen (2,5) needs 3 more electrons, so it shares one pair with each of three hydrogen atoms. That makes 3 bonding pairs plus 1 lone pair on the nitrogen: 8 electrons in total.

3Double and triple bonds

Some atoms share more than one pair.

• In an oxygen molecule, O₂, each O atom (2,6) needs 2 electrons, so the atoms share two pairs: a double bond, O=O.
• In carbon dioxide, CO₂, carbon needs 4 and each oxygen needs 2, so carbon forms a double bond with each oxygen: O=C=O.
• In a nitrogen molecule, N₂, each N atom (2,5) needs 3, so they share three pairs: a triple bond, N≡N. The triple bond is very strong, which is why the nitrogen that makes up 78% of the air is so unreactive, and why making fertiliser from it needs a large factory with high pressure and a catalyst.

4Valency

The valency of an element is the number of bonds its atom usually forms. For simple molecules it equals the number of electrons the atom needs to fill its outer shell.

5Properties of simple molecular substances

The covalent bonds inside each molecule are strong. But the forces between separate molecules, the intermolecular forces, are weak. When a simple molecular substance melts or boils, only these weak forces are overcome; the covalent bonds do not break. So:

• They have low melting and boiling points. Many are gases or liquids at room temperature (methane boils at −162 °C; water at 100 °C).
• They do not conduct electricity, because the molecules have no overall charge and there are no free electrons or ions.
• Many are insoluble in water but dissolve in organic solvents. Some, such as sugar, ethanol and ammonia, do dissolve in water.

Try it: open Build a Molecule (version 1.0.23) and build H₂O, NH₃, CO₂ and CH₄ from single atoms. Count the bonds on each atom and compare them with the valency table.

Practice

Try each item before you read the solution.