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Topic 2 · 2.4–2.7

Bonding and structure

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe how atoms form positive ions (cations) and negative ions (anions), and state that an ionic bond is a strong electrostatic attraction between them.
  • Draw dot-and-cross diagrams for ionic compounds of Group I and Group VII; for any metal and non-metal EXTENDED.
  • Describe and explain the properties of ionic compounds; describe the giant ionic lattice EXTENDED.
  • Draw dot-and-cross diagrams for H2, Cl2, H2O, CH4, NH3 and HCl; also CH3OH, C2H4, O2, CO2 and N2 EXTENDED.
  • Describe and explain the properties of simple molecular substances, using weak intermolecular forces EXTENDED.
  • Describe diamond and graphite and relate their structures to their uses; describe silicon(IV) oxide EXTENDED.
  • Describe metallic bonding and explain the conductivity, malleability and ductility of metals EXTENDED.

📚The chemistry

Ionic bonding

Metal atoms lose their outer electrons to form positive ions (cations); non-metal atoms gain electrons to form negative ions (anions). Both end up with a full outer shell. The ionic bond is the strong electrostatic attraction between the oppositely charged ions. In a dot-and-cross diagram, show one atom’s electrons as dots and the other’s as crosses, put each ion in square brackets, and write its charge outside the bracket.

Dot-and-cross diagram of sodium chloride. On the left, a sodium ion in square brackets with a plus charge, electron shells 2 and 8 shown as crosses. On the right, a chloride ion in square brackets with a minus charge, shells 2, 8 and 8 shown as dots, with one cross in the outer shell: the electron transferred from sodium.
Na (2,8,1) gives its outer electron to Cl (2,8,7): Na+ is 2,8 and Cl− is 2,8,8. The transferred electron keeps its cross.

Ionic compounds have high melting and boiling points, and they conduct electricity when molten or in aqueous solution, but not as solids. EXTENDED They form a giant lattice: a regular arrangement of alternating positive and negative ions. Many strong ionic bonds must be broken to melt it, which takes a lot of energy. In the solid the ions are fixed in place; when molten or dissolved they are free to move and carry charge.

A layer of a sodium chloride lattice: a square grid of alternating small sodium ions and larger chloride ions, each ion surrounded by ions of the opposite charge.
A layer of the NaCl lattice: every ion is surrounded by ions of the opposite charge. The pattern repeats in all three dimensions.

EXTENDED Work out the formula from the charges so that they cancel: Mg2+ and O2− give MgO; Al3+ and O2− give Al2O3 (\( 2 \times 3 = 3 \times 2 \)); Ca2+ and Cl− give CaCl2.

Covalent bonding

A covalent bond is a shared pair of electrons between two non-metal atoms, which gives each atom a noble-gas configuration. In diagrams, overlap the outer shells and put the shared pairs in the overlap. A double bond is two shared pairs (as in O2, C2H4 and CO2); a triple bond is three (N2). Check each atom: hydrogen should have 2 outer electrons, every other atom 8.

Two dot-and-cross diagrams showing outer shells only. Water: an oxygen shell overlapping two hydrogen shells, each overlap holding one dot and one cross, with two further pairs of dots on the oxygen. Carbon dioxide: a carbon shell overlapping two oxygen shells, each overlap holding two crosses and two dots (a double bond), with two lone pairs of dots on each oxygen.
H2O (two single bonds) and CO2 (two double bonds) EXTENDED. Count: O has 8 outer electrons in both; C has 8; each H has 2.

Simple molecular substances (H2O, CO2, CH4…) have low melting and boiling points and do not conduct. EXTENDED The covalent bonds inside each molecule are strong, but the forces between molecules (intermolecular forces) are weak, and only those are overcome on melting or boiling. There are no ions or free electrons to carry charge.

Giant covalent structures

Two structures. Diamond: each carbon atom covalently bonded to four others in a rigid three-dimensional tetrahedral network. Graphite: flat layers of hexagonal rings, each carbon bonded to three others, with weak forces between the layers shown dashed and a delocalised electron from each carbon.
Diamond: 4 bonds per carbon, rigid in 3D. Graphite: 3 bonds per carbon in layers, with one delocalised electron each and weak forces between layers.
  • Diamond: every carbon atom is covalently bonded to four others in a giant 3D network ⇒ very hard, very high melting point, does not conduct ⇒ used in cutting tools.
  • Graphite: each carbon bonded to three others in flat layers; the fourth electron is delocalised; weak forces between layers ⇒ the layers slide (used as a lubricant) and it conducts electricity (used as an electrode).
  • EXTENDED Silicon(IV) oxide, SiO2: each silicon bonded to four oxygens and each oxygen to two silicons in a giant network ⇒ like diamond, hard, very high melting point and a non-conductor.

Metallic bonding EXTENDED

Metallic bonding is the electrostatic attraction between the positive ions in a giant metallic lattice and a ‘sea’ of delocalised electrons. The delocalised electrons can move through the structure, so metals conduct electricity; the layers of ions can slide over each other without breaking the bonding, so metals are malleable and ductile.

A regular grid of positive metal ions surrounded by randomly scattered delocalised electrons, labelled a sea of delocalised electrons.
Positive ions in a lattice, held together by a sea of delocalised electrons.

✏️Worked example

(a) Describe how sodium and chlorine atoms form sodium chloride. [3] (b) EXTENDED Explain why sodium chloride has a high melting point and why it conducts electricity when molten but not when solid. [4] (c) EXTENDED Explain why methane, CH4, has a low boiling point even though its C–H bonds are strong. [2]

(a) A sodium atom (2,8,1) loses its outer electron to a chlorine atom (2,8,7), forming Na+ (2,8) and Cl− (2,8,8). The oppositely charged ions are held together by strong electrostatic attraction: an ionic bond.

(b) It is a giant lattice of oppositely charged ions with many strong ionic bonds, which need a lot of energy to break. In the solid the ions cannot move; when molten they are free to move and carry charge.

(c) Boiling only overcomes the weak intermolecular forces between the molecules, not the covalent bonds within them, so little energy is needed.

Check it. Every “explain the property” answer names the particles (ions, molecules, electrons) and the force (ionic bonds, intermolecular forces), and says whether it is strong or weak.
“Electrons carry the current in molten NaCl.” In ionic compounds it is the ions that move. Delocalised electrons carry the current only in metals and graphite. And “breaking covalent bonds” is wrong for boiling a simple molecular substance.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which substance contains ionic bonds? A: HCl. B: CH4. C: MgCl2. D: Cl2.
C. A metal combined with a non-metal; the others are all non-metals only (covalent).
2. (Theory.) In a molecule of ammonia, NH3, state the number of bonding pairs and non-bonding (lone) pairs of electrons around nitrogen. [2]
3 bonding pairs (one with each H) and 1 lone pair: 8 electrons around N in total.
3. (Theory.) Explain why diamond is used in cutting tools and graphite is used as a lubricant. [4]
In diamond each carbon is covalently bonded to four others in a rigid giant structure, so it is extremely hard. In graphite the carbon atoms are in layers held by weak forces, so the layers slide over each other easily.
4. (Theory.) EXTENDED Using the charges on the ions, write the formulae of aluminium oxide and calcium chloride. [2]
Al3+ and O2−: Al2O3 (charges +6 and −6). Ca2+ and Cl−: CaCl2.
5. (Theory.) EXTENDED Draw a dot-and-cross diagram for nitrogen, N2, showing outer electrons only. How many shared pairs are there? [2]
Two overlapping N shells with three shared pairs (three dots and three crosses) in the overlap, and one lone pair on each N: a triple bond. Each N has 8 outer electrons.
6. (Theory.) EXTENDED Explain why copper conducts electricity and can be drawn into wires. [3]
Metallic bonding: a lattice of positive ions in a sea of delocalised electrons. The delocalised electrons move through the metal and carry charge. The layers of ions can slide over each other while the attraction to the electron sea remains, so the metal is ductile.
7. (Theory.) EXTENDED Silicon(IV) oxide has a very high melting point and does not conduct electricity. Explain these properties. [3]
It is a giant covalent structure (each Si bonded to four O, each O to two Si): melting breaks many strong covalent bonds, needing a lot of energy. There are no free ions or delocalised electrons to carry charge.
8. (Multiple choice.) Which pair of properties describes a simple molecular substance? A: high melting point, conducts when molten. B: low melting point, does not conduct. C: high melting point, conducts when solid. D: low melting point, conducts when molten.
B. Weak forces between molecules; no mobile charged particles.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Chemistry — interactive dot-and-cross diagram builders
  • ChemTube3D — rotatable 3D structures of NaCl, diamond, graphite and SiO2