Covalent bonding occurs when two non-metal atoms share pairs of electrons to achieve full outer shells. Each shared pair of electrons constitutes a covalent bond, resulting in the formation of molecules or giant covalent structures. Each atom must share 1 electron each to form one covalent bond - so each covalent bond is made of 2 electrons.

The term "covalent" comes from "co-" (together) and "valent" (related to valence electrons). The electrostatic attraction in covalent bonding is between the shared pair(s) of electrons and the nuclei of the bonded atoms.

Simple Molecular Structures

Simple molecular substances consist of small molecules held together by strong covalent bonds. They contain a discrete number of atoms within their structure, e.g. water (H₂O):

• Each hydrogen atom shares one electron with the oxygen atom.
• Oxygen shares one of its electrons with each hydrogen, forming two covalent bonds.

Simple molecules are typically around 0.1 nanometres in size.

Properties of simple molecular substances:

• They have low melting and boiling points because little energy is required to overcome weak intermolecular forces of attraction between molecules
• They can be in the gas, liquid, or solid state depending on the strength of the intermolecular forces, but most are often in the liquid or gas state at room temperature and pressure
• They do not conduct electricity because there are no free-moving charge carriers (have neither delocalised electrons or free-moving ions)
• Most molecular substances are insoluble or only just soluble in water, but those that do dissolve often react with the water

Giant Covalent Structures

Giant covalent substances are made up of a vast number of atoms bonded together by covalent bonds, forming a giant lattice structure. Diamond, graphite and graphene are all giant structures that are allotropes of carbon.

Diamond (C)

• An allotrope of carbon
• Each carbon atom forms four covalent bonds to other carbon atoms, creating a very hard, 3D tetrahedral lattice
• High melting point as lots of energy required to break strong covalent bonds between carbon atoms
• Does not conduct electricity (no free-moving electrons or ions)
• Diamond is the hardest natural material and is used in cutting tools

Graphite (C)

• An allotrope of carbon
• Each carbon atom forms three covalent bonds to other carbon atoms, creating layers of hexagonal rings
• High melting point as lots of energy required to break strong covalent bonds between carbon atoms
• Layers are held together by weak forces, allowing them to slide over each other
• Conducts electricity due to delocalised electrons
• Used in pencils and as a lubricant due to its layered structure

Graphene (C) is a single layer of graphite, where each carbon atom forms three covalent bonds to other carbon atoms, creating a single layer of hexagonal rings. It is:

• An allotrope of carbon
• Very strong
• Lightweight and flexible
• A good electrical conductor
• Used in electronics, sensors, and composite materials

Silicon dioxide (SiO₂)

• Each silicon atom bonds with four oxygen atoms, creating a hard, 3D tetrahedral lattice
• High melting point as lots of energy required to break strong covalent bonds between silicon and oxygen atoms
• Found as quartz or in sand

Polymers

Polymers are large molecules made of repeating units called monomers, connected by covalent bonds. They are macromolecular structures, but not classed as giant covalent structures.

Polymers can be very long (thousands or millions of atoms). Their physical properties depend on the nature of the monomers and the conditions under which the polymer is made.

Poly(ethene) is made from repeating ethene (C₂H₄) units. It is a very widely used polymer, and is mostly used for plastic bags and bottles.

Fullerenes and Nanotubes

Fullerenes are molecules made entirely of carbon (another allotrope), taking the form of hollow spheres, ellipsoids, or tubes. A common fullerene is C₆₀ (buckminsterfullerene), which looks like a football and is sometimes called a buckyball.

Properties of spherical fullerenes:

• They have low melting points as little energy is needed to overcome the weak forces between molecules
• They have delocalised electrons within their structure, but these cannot pass from molecule to molecule (so cannot conduct electricity)
• They can act as lubricants as there are weak intermolecular forces between molecules, allowing them to slide around each other

Nanotubes, a type of cylindrical fullerene, have high length-to-diameter ratios and have a high tensile strength due to many strong covalent bonds. They can conduct electricity as delocalised electrons can move along the tube. Nanotubes are used in nanotechnology, electronics, and materials science.

Dot-and-Cross Diagrams

Dot-and-cross diagrams are a useful way to represent the transfer or sharing of electrons in chemical bonding. They show the outer shell electrons of atoms as crosses (×) or dots (•). This visual representation helps in understanding how atoms end up with full outer shells through bonding.

Only the outer shell electrons are involved in chemical bonding, so inner shells aren't often drawn for these diagrams.

Steps to draw dot-and-cross diagrams

1. Determine the number of electrons in the outer shell of each atom.
2. Decide whether the atoms will transfer or share electrons to achieve full outer shells.
3. Decide which outer shell electrons of one atom will be represented with dots and which one with crosses.
4. Draw the dot-and-cross diagram
5. Double check each atom has access to 8 electrons (or 2 if it is hydrogen)
6. For ionic compounds, include the charges on the resulting ions.

Bonding Models