Covalent Bonding

Definition of Covalent Bonding

A covalent bond is formed when two atoms share a pair of electrons. This sharing allows each atom to achieve a full outer shell, making the atoms more stable. Covalent bonding usually occurs between non-metal atoms. The atoms join together to form molecules, which are groups of atoms held together by covalent bonds.

Formation of Covalent Bonds

Atoms form covalent bonds by sharing electrons in their outer shells to complete their valence shells, typically aiming for eight electrons (the octet rule), or two electrons for hydrogen (which only needs a full shell of two).

There are different types of covalent bonds depending on how many pairs of electrons are shared:

• Single bond: sharing one pair of electrons (2 electrons)
• Double bond: sharing two pairs of electrons (4 electrons)
• Triple bond: sharing three pairs of electrons (6 electrons)

Dot and cross diagrams are used to show covalent bonding. They represent the electrons from each atom with different symbols (dots or crosses) to illustrate which electrons are shared and which belong to each atom.

For example, in a water molecule (H₂O), each hydrogen atom shares one electron with the oxygen atom, forming two single covalent bonds. Oxygen has six electrons in its outer shell and needs two more to complete its octet, so it shares one electron with each hydrogen.

Example: Drawing a dot and cross diagram for methane (CH₄)

Carbon has 4 electrons in its outer shell and needs 4 more to complete its octet. Each hydrogen atom has 1 electron and needs 1 more to fill its outer shell (which only needs 2 electrons). Carbon shares one electron with each hydrogen, forming four single covalent bonds.

The diagram shows carbon in the centre with four pairs of shared electrons (each pair consisting of one dot and one cross) between carbon and each hydrogen atom.

Properties of Covalent Substances

Covalent substances have distinct properties due to the nature of the bonds and the structure of the molecules:

• Low melting and boiling points: Covalent bonds within molecules are strong, but the forces between molecules (intermolecular forces) are weak. This means less energy is needed to separate the molecules, resulting in low melting and boiling points.
• Poor electrical conductivity: Covalent molecules do not have free ions or electrons to carry charge, so they do not conduct electricity in solid or liquid states.
• Physical state at room temperature: Many covalent substances are gases or liquids at room temperature because of the weak forces between molecules.

Note: These properties apply to simple molecular covalent substances, not to giant covalent structures like diamond or graphite, which have very different properties.

For example, oxygen (O₂) and hydrogen (H₂) are gases, while water (H₂O) is a liquid at room temperature.

Examples of Covalent Molecules

Common examples of molecules formed by covalent bonding include:

• Hydrogen (H₂): Two hydrogen atoms share one pair of electrons, forming a single covalent bond.
• Oxygen (O₂): Two oxygen atoms share two pairs of electrons, forming a double covalent bond.
• Water (H₂O): One oxygen atom shares electrons with two hydrogen atoms, forming two single covalent bonds.
• Methane (CH₄): One carbon atom shares electrons with four hydrogen atoms, forming four single covalent bonds.

Example: Understanding the bonding in oxygen (O₂)

Each oxygen atom has six electrons in its outer shell and needs two more to complete the octet. They share two pairs of electrons, forming a double bond. The dot and cross diagram shows two shared pairs of electrons between the oxygen atoms, and each oxygen atom has four other electrons not involved in bonding.

For instance, consider a hydrogen molecule (H₂): two hydrogen atoms share one pair of electrons forming a single covalent bond, which stabilizes both atoms.