Skip to main content
SingaporeCombined ScienceSyllabus dot point

Why do atoms join together, and what is the difference between ionic and covalent bonding?

Explain why atoms form bonds, describe ionic bonding as the transfer of electrons and covalent bonding as the sharing of electrons, and relate bonding to simple properties

A focused N(A)-Level answer on bonding. Why atoms bond, ionic bonding by transferring electrons, covalent bonding by sharing, and how bonding links to melting point and conductivity.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to explain why atoms bond, to describe ionic bonding as a transfer of electrons and covalent bonding as the sharing of electrons, and to link the type of bonding to simple properties such as melting point and electrical conductivity. The central idea is that atoms bond to gain a full, stable outer electron shell.

The answer

Why atoms bond

Atoms bond to achieve a full outer electron shell, the same stable arrangement as the noble gases. They can do this in two main ways: by transferring electrons (ionic bonding) or by sharing electrons (covalent bonding).

Ionic bonding

Ionic bonding happens between a metal and a non-metal. Electrons are transferred from the metal atom to the non-metal atom:

  • the metal atom loses electrons and becomes a positive ion,
  • the non-metal atom gains those electrons and becomes a negative ion,
  • the oppositely charged ions attract strongly and pack into a giant lattice.

For example, sodium gives its outer electron to chlorine, forming Na+\text{Na}^{+} and Cl−\text{Cl}^{-} ions held together in sodium chloride.

Covalent bonding

Covalent bonding happens between non-metal atoms. Instead of transferring electrons, the atoms share pairs of electrons so that each gets a full outer shell. Each shared pair is one covalent bond. Water (H2O\text{H}_2\text{O}), oxygen (O2\text{O}_2) and methane (CH4\text{CH}_4) are all covalent.

Bonding and properties

The type of bonding explains the properties:

  • ionic compounds have high melting points (strong attraction across a giant lattice), and they conduct electricity when molten or dissolved (the ions are then free to move),
  • simple covalent substances have low melting points (weak forces between small molecules), and they do not conduct electricity (no free-moving charged particles).

Examples in context

Example 1. Why salt dissolves and conducts but sugar does not. Sodium chloride is ionic, so when it dissolves the ions separate and can carry a current. Sugar is covalent, so it dissolves as whole neutral molecules with no charged particles, and its solution does not conduct.

Example 2. Why diamond is so hard but candle wax is soft. Diamond is a giant covalent structure with every carbon atom strongly bonded to four others, making it extremely hard. Candle wax is made of small molecules with weak forces between them, so it is soft and melts at a low temperature.

Try this

  • Cue. State the type of bonding in potassium chloride (a metal and a non-metal). Ionic bonding.
  • Cue. Explain why molten sodium chloride conducts electricity. The ions are free to move and carry the charge.
  • Cue. State why carbon dioxide has a low melting point. It is made of small molecules held by weak forces that need little energy to separate.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksSodium (2,8,12, 8, 1) reacts with chlorine (2,8,72, 8, 7) to form sodium chloride. (a) Describe what happens to the electrons. (b) State the charge on each ion formed.
Show worked answer →

(a) Sodium loses its one outer electron and chlorine gains that electron, so an electron is transferred from sodium to chlorine. Both then have full outer shells.

(b) Sodium becomes a Na+\text{Na}^{+} ion (charge +1+1) and chlorine becomes a Cl−\text{Cl}^{-} ion (charge −1-1).

What markers reward: transfer of one electron from sodium to chlorine, both reaching full outer shells, and the correct ion charges +1+1 and −1-1.

Original3 marks(a) Describe the bonding in a water molecule, H2O\text{H}_2\text{O}. (b) Explain why simple covalent substances usually have low melting points.
Show worked answer →

(a) Water has covalent bonds: each hydrogen atom shares a pair of electrons with the oxygen atom.

(b) Simple covalent substances are made of small molecules held to each other by weak forces between molecules. Only a little energy is needed to separate the molecules, so the melting point is low.

What markers reward: covalent bonding as shared electron pairs, and low melting points explained by weak forces between molecules (not breaking the covalent bonds).

Related dot points