Covalent bonding as the sharing of electrons between non-metals, simple molecular substances, and their properties.

What this dot point is asking

WJEC Double Award Unit 5 wants you to describe covalent bonding as the sharing of electrons, simple molecular substances, and their properties.

How covalent bonding forms

Each atom shares electrons so that it gains a full outer shell. For example, in a hydrogen molecule ($\text{H}_2$), each hydrogen atom shares its one electron, making a shared pair so both have a full first shell. In water ($\text{H}_2\text{O}$), the oxygen shares a pair with each of two hydrogen atoms.

Simple molecules

Many covalent substances are made of small molecules with a fixed number of atoms, such as:

• Hydrogen ($\text{H}_2$), oxygen ($\text{O}_2$) and chlorine ($\text{Cl}_2$),
• Water ($\text{H}_2\text{O}$), carbon dioxide ($\text{CO}_2$) and methane ($\text{CH}_4$).

The atoms within each molecule are held by strong covalent bonds, but the forces between different molecules (intermolecular forces) are weak.

Properties of simple molecular substances

Why the covalent bonds do not break on melting

A common exam point is that when a simple molecular substance melts or boils, it is the weak forces between the molecules that are overcome, not the strong covalent bonds inside each molecule. The molecules stay intact and simply move apart. This is why these substances have low melting points despite the covalent bonds themselves being strong, and it explains why, for example, water turns to steam as whole $\text{H}_2\text{O}$ molecules, not separate hydrogen and oxygen atoms.

Drawing covalent molecules

Covalent bonding is shown with dot-and-cross diagrams, drawing the outer electrons of each atom and showing the shared pairs where the atoms overlap. In water, for example, the oxygen shares one pair of electrons with each hydrogen, giving two shared pairs (the bonds). A single shared pair is a single bond; some molecules have double bonds (two shared pairs), such as oxygen ($\text{O}_2$) and carbon dioxide ($\text{CO}_2$). Being able to draw the shared electrons and count the bonds in a small molecule is a common exam skill.

Comparing ionic and covalent bonding

It helps to contrast the two main types of bonding. Ionic bonding is between a metal and a non-metal and involves transferring electrons to form charged ions. Covalent bonding is between non-metals and involves sharing electrons. Ionic compounds form giant lattices with high melting points and conduct when molten; simple covalent substances form small molecules with low melting points and do not conduct. Knowing which type of bonding to expect from the elements involved (metal plus non-metal versus non-metal plus non-metal) is the first step in many bonding questions.

Try this

Q1. What is shared in a covalent bond? [1 mark]

• Cue. A pair of electrons.

Q2. Why do simple molecular substances have low melting points? [1 mark]

• Cue. The forces between the molecules are weak and need little energy to overcome.