Ions form by electron transfer
An ion is a charged particle formed when an atom loses or gains electrons. Metals lose electrons to form positive ions, e.g. (Groups 1, 2, 3 give , , ). Non-metals gain electrons to form negative ions, e.g. (Groups 7, 6, 5 give , , ). The charge equals the electrons transferred, giving each ion a full outer shell (a noble-gas arrangement).
Ionic bond and the giant lattice
The mark-scheme definition of an ionic bond has two elements: (1) electrostatic attraction, (2) between oppositely charged ions. Ionic compounds are not molecules — they form a giant ionic lattice: a regular, repeating 3-D arrangement in which each ion is surrounded by ions of opposite charge. The attractions act in all directions, so no single molecule of exists. Bond strength rises with larger charges and smaller ions.
Properties follow from the giant lattice
High melting/boiling points: many strong attractions between oppositely charged ions must be overcome, needing a large amount of energy to separate them; larger charges (e.g. ) melt higher than . Conductivity: an ionic compound conducts only when its ions are free to move and carry charge. Solid — ions locked in fixed positions, does NOT conduct; molten or aqueous — the lattice breaks up, the ions are mobile, so it DOES conduct.
Drawn from real examiner reports.
Dot-and-cross must show the transfer
A dot-and-cross diagram must SHOW the transfer: the metal loses its outer electron(s) and the non-metal gains them (dots vs crosses). Common faults: redrawing the two neutral atoms unchanged (no transfer); missing inner shells when required; and missing or wrong charges, written in square brackets, e.g. .
June 2024 Paper 1CR Q9(b)(ii): marks lost for not showing inner shells, for missing or incorrect charges, and for candidates who just redrew the two atoms with no electron transfer — copying the atoms gains no transfer mark.
Ionic bond ≠ sharing or joining
The definition-precision trap. "Atoms join", "atoms share electrons" or "opposite charges attract" alone do not score. The mark-scheme answer needs BOTH elements: electrostatic attraction between oppositely charged ions. Losing or gaining electrons describes how the ions form — it is not the bond itself. State both parts every time.
June 2024 Paper 1CR: a structure question credited "giant ionic structure with large electrostatic forces between oppositely charged ions" but rejected "a simple covalent bond".
"The ions move" is not enough
To explain why an ionic compound conducts only when molten or aqueous, you must say the ions are free to move and carry charge. Vague answers — "the ions move", "it carries charge", "electrons flow" — do not score. A solid does not conduct because its ions are held in fixed positions. The carriers here are IONS, not delocalised electrons (that is metallic bonding).
November 2024 Paper 2C: candidates lost marks for saying "the ions moved" or that the substance "carried charge" without stating that the charge carriers were free to move.
Ionic is giant, not simple covalent
Do not treat an ionic compound as simple covalent. When comparing with a molecular substance such as , you overcome its weak intermolecular forces — you do NOT "break the covalent bonds". Ionic compounds like are giant lattices held by strong electrostatic forces, so they melt much higher than molecular substances.
June 2024 Paper 1CR Q9(b)(iii): comparing melting points, candidates wrongly treated MgCl2 as covalent and described breaking the covalent bonds in HCl rather than overcoming its weak intermolecular forces.
Match the charge to electrons moved
The size of the charge equals the number of electrons transferred. Lose 2 electrons and the ion is (, not ); lose 1 and it is . Gain 1 and it is (chlorine becomes ); gain 2 and it is . When two electrons pass to two chlorine atoms, the metal ion is and each chlorine — a commonly confused count.
June 2024 Paper 1CR Q9(b)(ii): when two electrons are transferred the metal ion is 2+ and each chlorine that gains one electron is 1- — candidates confused the counts.
A giant lattice, not molecules
An ionic compound is a giant ionic lattice, not separate molecules — there is no single "molecule of ". The formula (e.g. ) gives the simplest whole-number RATIO of ions, not the atoms in a molecule. Call the structure a "giant lattice", not just "giant", and never describe individual ionic molecules.
(common Edexcel chemistry error)
Balance charges to build a formula
Make the total positive and negative charges equal: find the smallest whole-number ratio of ions that cancels. with gives ; put a polyatomic ion in brackets when used more than once, e.g. .
Explanations need a linked chain
Property questions want a chain of reasoning, not a list of facts. For melting points, link cause to effect: larger ionic charges → stronger electrostatic attractions → more energy to separate the ions → higher melting point. Each step must follow from the last.
State the condition for conductivity
For conductivity, name the state and its reason. Solid: ions held in fixed positions, cannot move, does not conduct. Molten or aqueous: lattice broken up, ions free to move and carry charge, so it conducts. Always include the full phrase "free to move and carry charge".
The charge on a simple ion follows the group:
| Group | Ion charge | Examples |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 5 | ||
| 6 | ||
| 7 |
Ions you must learn by name (4CH1 spec list):
| Ion | Formula | Ion | Formula |
|---|---|---|---|
| Silver | Hydrogen | ||
| Copper(II) | Hydroxide | ||
| Iron(II) | Ammonium | ||
| Iron(III) | Carbonate | ||
| Lead(II) | Nitrate | ||
| Zinc | Sulfate |
What charge do ions of Groups 1, 2, 3 and Groups 5, 6, 7 have?
Magnesium is in Group 2 and chlorine is in Group 7.
Write the formula of the ionic compound magnesium chloride.
