Covalent Bonding and Molecules

A covalent bond is a shared pair of electrons between two atoms. Both atoms contribute one electron to the shared pair.

Why Do Atoms Share?

Non-metal atoms need to gain electrons to fill their outer shells, but neither atom is willing to give up electrons. The solution is to share — both atoms count the shared electrons as part of their outer shell.

The shared pair of electrons is attracted to the nuclei of both atoms, holding them together.

Dot-and-cross diagrams show how outer-shell electrons are arranged in a covalent bond. One atom's electrons are shown as dots (•), the other's as crosses (×).

Simple Examples

Hydrogen ($\text{H}_2$):

• Each H atom has 1 electron, needs 2 for a full shell
• They share one pair of electrons → single bond
• Each hydrogen now has 2 electrons (full first shell)

Chlorine ($\text{Cl}_2$):

• Each Cl atom has 7 outer electrons, needs 8
• They share one pair → single bond
• Each Cl now has 8 outer electrons

Water ($\text{H}_2\text{O}$):

• Oxygen has 6 outer electrons, needs 8
• Each hydrogen has 1 outer electron, needs 2
• Oxygen shares one pair with each of two hydrogen atoms
• Oxygen gets 8 outer electrons ✓, each H gets 2 ✓

Ammonia ($\text{NH}_3$):

• Nitrogen has 5 outer electrons, needs 8
• Each hydrogen has 1 outer electron, needs 2
• Nitrogen shares one pair with each of three hydrogens
• N has 8 outer electrons ✓, each H has 2 ✓
• N has one lone pair (non-bonding pair)

Methane ($\text{CH}_4$):

• Carbon has 4 outer electrons, needs 8
• Each hydrogen has 1, needs 2
• Carbon shares one pair with each of four hydrogens

Oxygen ($\text{O}_2$): Each O has 6 outer electrons, needs 8. Two pairs are shared → double bond.

Nitrogen ($\text{N}_2$): Each N has 5 outer electrons, needs 8. Three pairs are shared → triple bond. This makes N₂ very unreactive (the triple bond is very strong).

Carbon dioxide ($\text{CO}_2$): Carbon shares two pairs with each oxygen → two double bonds: O=C=O

Substances made of small covalent molecules are called simple molecular substances.

Important Distinction

• The covalent bonds within each molecule are strong
• The intermolecular forces between molecules are weak
• When a simple molecular substance melts or boils, it is the weak intermolecular forces that are overcome, NOT the covalent bonds

Properties

Examples of Simple Molecular Substances

Water ($\text{H}_2\text{O}$), carbon dioxide ($\text{CO}_2$), methane ($\text{CH}_4$), oxygen ($\text{O}_2$), hydrogen ($\text{H}_2$), glucose ($\text{C}_6\text{H}_{12}\text{O}_6$)

Some covalent substances don't form small molecules — instead, they form giant covalent structures (also called macromolecules) where billions of atoms are bonded in a continuous network.

Diamond

• Each carbon is bonded to 4 other carbons in a tetrahedral arrangement
• Very hard (hardest natural substance) — strong bonds in all directions
• Very high melting point ($3550°\text{C}$) — many strong covalent bonds to break
• Does not conduct electricity — no free electrons or ions
• Used in cutting tools and jewellery

Graphite

• Each carbon is bonded to 3 other carbons in flat layers (hexagonal rings)
• The 4th electron from each carbon is delocalised (free to move between layers)
• Layers are held together by weak intermolecular forces → layers slide easily
• Soft and slippery — used as a lubricant and in pencil leads
• High melting point — strong covalent bonds within layers
• Conducts electricity — delocalised electrons can move along the layers

Graphene

• A single layer of graphite — one atom thick
• Extremely strong and light
• Excellent electrical conductor
• Applications: electronics, composite materials, touchscreens, batteries

Silicon Dioxide ($\text{SiO}_2$)

• Each silicon bonded to 4 oxygens, each oxygen bonded to 2 silicons
• Very high melting point ($1710°\text{C}$)
• Very hard — used in glass and ceramics
• Does not conduct electricity

Polymers are very large covalent molecules made of repeating units. Examples include poly(ethene) and nylon.

• Relatively strong intermolecular forces between long chains
• Solid at room temperature
• Don't conduct electricity
• Properties depend on the type of polymer and cross-links between chains

• Never say "covalent bonds break" when a substance melts/boils — say "intermolecular forces are overcome"
• In dot-and-cross diagrams, show ALL outer electrons, including lone pairs
• For giant covalent structures, emphasise "many strong covalent bonds" when explaining high melting points
• Know the differences between diamond, graphite, and graphene — this is very commonly tested

• Covalent bonds form when non-metal atoms share pairs of electrons
• Simple molecular substances have weak intermolecular forces → low melting/boiling points, don't conduct electricity
• Giant covalent structures (diamond, graphite, SiO₂) have many strong covalent bonds → very high melting points
• Graphite conducts electricity due to delocalised electrons
• When substances melt/boil, intermolecular forces are overcome, not covalent bonds