Ions and Ionic Bonds

Sodium chloride pays the bills on Paper 4: describe the bonding (2-3 marks), draw or interpret a dot-and-cross diagram (2 marks), then explain melting point and conductivity (2-3 marks more). The same question skeleton has run for years with the compounds swapped. Master one ionic compound properly and you have mastered the question. This page builds it from the atomic structure you already know.

How ions form (Core)

Atoms react to achieve a full outer shell: the noble-gas configuration. Metal atoms lose their outer electrons to become positive ions (cations); non-metal atoms gain electrons to become negative ions (anions). Sodium (2,8,1) loses one electron to form Na+ (2,8); chlorine (2,8,7) gains one to form Cl− (2,8,8). The charge equals the number of electrons lost or gained, which is why group number predicts charge: Group I gives +1, Group II +2, Group VI −2, Group VII −1.

An ionic bond is the strong electrostatic attraction between the oppositely charged ions. Core candidates describe this for Group I metals with Group VII non-metals; Supplement candidates extend it to any pairing, including compounds of Group II and Group VI, and ions of metals with variable charge like Fe2+ and Fe3+.

Dot-and-cross diagrams in words (Core)

A dot-and-cross diagram for NaCl shows a sodium ion drawn as its symbol in square brackets with a + charge outside, its outer shell now empty (or shown as the full 2,8 inner arrangement), and a chloride ion in square brackets with a − charge, its outer shell holding eight electrons: seven drawn as dots (chlorine’s own) and one as a cross (the electron transferred from sodium). The brackets and charges are compulsory; the transferred electron must be distinguishable from the others.

For magnesium chloride the diagram needs one Mg2+ and two Cl− ions, each chloride showing one crossed electron, and for magnesium oxide, both transferred electrons end up on the one O2− ion. Check the formula first using the swap-and-drop method from chemical formulae; the diagram must match it.

The giant ionic lattice and its properties (Core, explained fully at Supplement)

Ionic compounds form a giant lattice: a regular, repeating arrangement of alternating positive and negative ions, held together by electrostatic attraction acting in all directions.

Property	Explanation
High melting and boiling points	Strong electrostatic forces between oppositely charged ions throughout the lattice need a lot of energy to overcome
Do not conduct when solid	Ions are held in fixed positions, so no mobile charged particles
Conduct when molten or aqueous	Ions are free to move and carry charge
Brittle, hard crystals	Regular lattice; layers shifted bring like charges together, which repel
Mostly soluble in water	Water molecules attract the ions out of the lattice

The conductivity rows do double duty: they are the entry requirement for electrolysis in topic 4. Contrast all of this with simple molecules and covalent bonds, where weak intermolecular forces produce low melting points and no conduction at all.

Worked exam question

Calcium reacts with chlorine to form calcium chloride. (a) Describe, in terms of electrons, what happens when a calcium atom reacts with chlorine atoms. (3) (b) Explain why calcium chloride has a high melting point. (2)

Model answer: (a) Each calcium atom loses two electrons (1) forming a Ca2+ ion; each chlorine atom gains one electron, so two chloride ions (Cl−) form per calcium (1); both ions reach a full outer shell / noble-gas configuration (1). (b) Strong electrostatic forces of attraction between oppositely charged ions (1) throughout the giant lattice require a large amount of energy to overcome (1).

Mark-by-mark: (a) wants transfer direction, the numbers (two lost, one gained each, two chlorines), and the resulting charges. Vague “electrons are shared” is a covalent answer and scores zero. (b) needs both the force named with its cause (oppositely charged ions) and the energy statement. “Ionic bonds are strong” without “electrostatic” or “between oppositely charged ions” usually drops a mark.

The mistakes that cost marks

1. Writing “shares electrons” for an ionic compound. Transfer is the word; sharing belongs to covalent bonding.
2. Dot-and-cross diagrams without brackets and charges, or with the transferred electron indistinguishable. Both omissions are penalised.
3. Explaining molten conduction with “free electrons”. In ionic compounds, the mobile particles are ions. Electrons answer the metals question, not this one.
4. “The molecules of NaCl…”: ionic compounds have no molecules. NaCl is a giant lattice of ions; the word molecule in an ionic answer flags a misconception examiners explicitly report.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”The sodium gives its electron away"	"The sodium atom transfers one electron to the chlorine atom, forming Na+ and Cl−"
"Ionic bonds are really strong so it’s hard to melt"	"Strong electrostatic attraction between oppositely charged ions requires a large amount of energy to overcome"
"It conducts when melted because electricity passes"	"When molten, the ions are free to move and carry the charge"
"It wants a full shell"	"The atom gains/loses electrons to achieve a noble-gas electronic configuration”

The Malaysia note

Bonding is taught mid-Year 10 in most Malaysian international schools, and by the May/June exam the dot-and-cross conventions (brackets, charges, crosses versus dots) have faded even when the concept hasn’t. The fix is drawing five compounds from scratch under time pressure, not re-reading notes. Diagram conventions are one of the quickest wins we deliver in a free trial lesson: 1-to-1, the examiner’s checklist takes twenty minutes to install permanently.

Test yourself

Answer all three before checking. The answers are hidden until you click.

Q1 (2 marks). Describe, in terms of electrons, what happens when magnesium reacts with oxygen to form magnesium oxide.

Show answer

• Each magnesium atom loses two electrons, forming an Mg2+ ion [1] • Each oxygen atom gains two electrons, forming an O2− ion [1]

Q2 (2 marks). Explain why solid potassium bromide does not conduct electricity but molten potassium bromide does.

Show answer

• In the solid, the ions are held in fixed positions in the lattice, so there are no mobile charged particles [1] • When molten, the ions are free to move and carry the charge [1]

Q3 (2 marks). Sodium chloride forms a giant ionic lattice. Describe this structure.

Show answer

• A regular, repeating arrangement of alternating positive (Na+) and negative (Cl−) ions [1] • Held together by strong electrostatic attraction (between oppositely charged ions) acting in all directions [1]