IGCSE Chemistry: ionic, covalent and metallic bonding

The four structures and their properties

Key ideas board by board

Cambridge 0620 Core asks for ion formation, the ionic bond as a strong electrostatic attraction between oppositely charged ions, dot-and-cross diagrams for Group I with Group VII compounds, and the properties of ionic compounds. Supplement extends dot-and-cross to any metal and non-metal ions (for example MgO, CaCl2), describes the giant lattice of alternating ions, and asks you to explain the properties. For covalent bonding, Core molecules are H2, Cl2, H2O, CH4, NH3 and HCl; Supplement adds CH3OH, C2H4, O2, CO2 and N2, which include double and triple bonds. Graphite and diamond are Core; silicon(IV) oxide and its similarity to diamond are Supplement. All of metallic bonding (2.7) is Supplement.

Edexcel 4CH1 requires ion charges to be known (Groups 1, 2, 3, 5, 6, 7, plus Ag+, Cu2+, Fe2+, Fe3+, Pb2+, Zn2+, H+, OH-, NH4+, CO3 2-, NO3-, SO4 2-), dot-and-cross diagrams for ionic compounds from Groups 1, 2, 3 with 5, 6, 7, and covalent molecules including O2, N2, CO2, ethene and ethane. It also asks why melting points of simple molecular substances generally increase with relative molecular mass (1.48) and how diamond, graphite and C60 fullerene structures affect properties (1.50). Metallic bonding (1.52C to 1.54C) is examined in Paper 2 only.

Worked example 1: magnesium oxide by dot and cross

Magnesium is 2,8,2 and oxygen is 2,6. Magnesium loses its two outer electrons to become Mg2+ with configuration 2,8. Oxygen gains two electrons to become O2- with configuration 2,8. In the diagram, draw magnesium's electrons as dots and oxygen's as crosses; the oxide ion's outer shell shows six crosses and two dots, in square brackets with the 2- charge outside. The magnesium ion is drawn with an empty outer shell (or with its full second shell) in brackets with 2+.

Explain the high melting point: MgO is a giant ionic lattice with strong electrostatic attractions between Mg2+ and O2- ions in all directions. A lot of energy is needed to overcome them. The attraction is stronger than in NaCl because the ions carry 2+ and 2- charges, which is why MgO melts at a much higher temperature.

Worked example 2: formulae from charges and a covalent molecule

Aluminium oxide: Al3+ and O2-. The lowest common multiple of 3 and 2 is 6, so two Al3+ ions (6+) balance three O2- ions (6-): Al2O3. Calcium nitrate: Ca2+ and NO3-, so two nitrate ions are needed: Ca(NO3)2. The brackets matter; CaNO32 is wrong.

Carbon dioxide: carbon (2,4) needs four more electrons, each oxygen (2,6) needs two. Carbon forms a double bond (two shared pairs) with each oxygen, O=C=O. In the dot-and-cross diagram each double bond shows two dots and two crosses in the overlap, and each oxygen keeps two lone pairs. Count: every atom ends with eight electrons in its outer shell.

Common mistakes

• Saying ionic compounds conduct when solid. The ions are fixed in the lattice; they must be free to move.
• Saying simple molecular substances have low melting points because covalent bonds are weak. Covalent bonds are strong; the intermolecular forces are weak.
• Saying metals conduct because ions move. It is the delocalised electrons that move.
• Writing "molecules" of sodium chloride. Ionic compounds are lattices of ions, not molecules.
• Explaining graphite's conductivity with free ions, or diamond's hardness with "strong intermolecular forces".
• Forgetting the charges and square brackets on ions in dot-and-cross diagrams.

Exam technique

Explain-the-property questions are marked on three ideas: name the structure, name the particles and the force between them, and link the strength of that force to energy. For example: "diamond is a giant covalent structure; each carbon atom forms four strong covalent bonds; a lot of energy is needed to break these bonds, so the melting point is high". Missing the structure name or the energy link costs a mark each.

In comparison questions (graphite versus diamond, NaCl versus HCl), write a sentence for each substance and use "whereas". Avoid "it": examiners need to know which substance you mean.

How one-to-one lessons help

Bonding questions reward precise vocabulary more than almost any other chemistry topic, and students often know the idea but name the wrong force. In lessons a tutor has the student draw dot-and-cross diagrams live on the shared whiteboard, catches missing charges and lone pairs immediately, and drills the three-part property explanation until it comes out right under time pressure.

Self-check

1. Draw dot-and-cross diagrams for NaCl, MgO, H2O, CH4, NH3 and (Extended or Edexcel) CO2 and N2.
2. Write formulae from ion charges, including brackets for polyatomic ions.
3. Explain why ionic compounds conduct only when molten or dissolved.
4. Explain why simple molecular substances have low melting points.
5. Compare the structures and uses of diamond and graphite.
6. Describe metallic bonding and explain conductivity and malleability.