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SingaporeCombined ScienceSyllabus dot point

How do atoms join together, and how does the type of bond explain a substance's properties?

Describe ionic and covalent bonding in terms of electron transfer and sharing, and relate bonding to the properties of ionic compounds and simple molecules

A focused answer to the O-Level Combined Science outcome on bonding. Ionic bonding by electron transfer, covalent bonding by sharing, and how the type of bonding explains melting points and conductivity.

Generated by Claude Opus 4.88 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe how ionic bonds form by transferring electrons and how covalent bonds form by sharing electrons, and to relate the type of bonding to a substance's properties such as melting point and electrical conductivity. The marks come from clear electron descriptions (transfer versus sharing) and from linking structure to properties.

The answer

Why atoms bond

Atoms bond to gain a full outer shell of electrons, which is a stable arrangement like that of the noble gases. They do this by transferring electrons (ionic bonding) or by sharing electrons (covalent bonding).

Ionic bonding

Ionic bonding occurs between a metal and a non-metal. The metal atom loses its outer electrons to form a positive ion; the non-metal atom gains them to form a negative ion. The oppositely charged ions attract strongly, and this electrostatic attraction is the ionic bond. For example, sodium gives one electron to chlorine to form Na+\text{Na}^{+} and Cl−\text{Cl}^{-}.

Covalent bonding

Covalent bonding occurs between non-metal atoms. The atoms share pairs of electrons so that each effectively gains a full outer shell. A shared pair is a single covalent bond. For example, two hydrogen atoms share a pair to form H2\text{H}_2, and oxygen and hydrogen share pairs in water, H2O\text{H}_2\text{O}.

Properties of ionic compounds

Ionic compounds form giant lattices of ions held by many strong forces, giving:

  • high melting and boiling points (a lot of energy breaks the many strong bonds),
  • conduction of electricity when molten or dissolved (ions free to move) but not when solid (ions fixed),
  • often soluble in water.

Properties of simple molecular substances

Simple covalent (molecular) substances have strong bonds within molecules but weak forces between molecules, giving:

  • low melting and boiling points (little energy separates the molecules),
  • usually no conduction of electricity (no free ions or electrons).

Examples in context

Example 1. Table salt versus sugar. Salt (sodium chloride) is ionic, so it has a high melting point and its solution conducts electricity. Sugar is molecular, so it melts and caramelises at a much lower temperature and its solution does not conduct, a direct demonstration of how bonding sets the properties.

Example 2. Why graphite conducts but diamond does not. Both are carbon, but in graphite each atom uses only three of its four outer electrons for bonds, leaving free electrons that carry charge. This shows that the arrangement of bonding electrons, not just the element, controls conduction.

Try this

Q1. State the type of bonding you expect between a metal and a non-metal. [1 mark]

  • Cue. Ionic bonding (electrons are transferred from the metal to the non-metal).

Q2. Explain why a simple molecular substance such as iodine has a low melting point. [2 marks]

  • Cue. The forces between the molecules are weak, so only a little energy is needed to separate them, giving a low melting point.

Q3. Explain why molten sodium chloride conducts electricity but solid sodium chloride does not. [2 marks]

  • Cue. When molten, the ions are free to move and carry charge; in the solid the ions are locked in the lattice and cannot move, so it does not conduct.

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 (proton number 1111) reacts with chlorine (proton number 1717) to form sodium chloride. (a) Describe how the bond forms in terms of electrons. (b) Give the charges on the two ions formed.
Show worked answer →

(a) Sodium has one outer electron and chlorine has seven. The sodium atom transfers its single outer electron to the chlorine atom, so both end up with full outer shells.

(b) The sodium atom loses an electron to become Na+\text{Na}^{+} (charge +1+1); the chlorine atom gains an electron to become Cl−\text{Cl}^{-} (charge −1-1). The oppositely charged ions attract in an ionic bond.

Markers reward the transfer of one electron from sodium to chlorine, full outer shells, and the correct ion charges +1+1 and −1-1.

Original4 marksExplain why sodium chloride has a high melting point and conducts electricity when molten but not when solid.
Show worked answer →

Sodium chloride is a giant lattice of oppositely charged ions held by strong electrostatic forces. A large amount of energy is needed to break these many strong forces, so the melting point is high.

When solid, the ions are fixed in the lattice and cannot move, so it does not conduct. When molten, the ions are free to move and carry charge, so it conducts electricity.

Markers reward the strong forces between many ions for the high melting point, and mobile ions when molten versus fixed ions when solid for the conduction.

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