Part 1: Ions, ionic bonding and metals

Separate the formation of charged particles from the attraction that holds a lattice together, then explain radius trends and evidence for mobile ions.

Electron transfer forms ions; attraction forms the ionic bond

An atom that loses electrons becomes a cation; an atom that gains electrons becomes an anion. Magnesium loses its two outer 3s electrons to become Mg²⁺, while oxygen gains two electrons to complete 2p and becomes O²⁻. The nucleus does not change.

Ionic bonding is the strong electrostatic attraction between oppositely charged ions. Electron transfer explains how ions form, but is not the bond itself. In solid magnesium oxide, each ion attracts several oppositely charged neighbours throughout a giant lattice; there are no isolated MgO molecules.

Build an ionic dot-and-cross diagram

Use dots for one atom’s original outer electrons and crosses for the other’s. For MgO, draw a bracketed Mg²⁺ ion and a separate bracketed O²⁻ ion. Oxygen’s outer shell contains six dots and two crosses, arranged as four pairs; the crosses represent magnesium’s transferred electrons. If showing only the original outer shells, magnesium has no electrons from its lost 3s shell to draw. If showing all shells, its remaining configuration is 2,8.

For CaCl₂, calcium loses two electrons and each chlorine gains one. Draw one Ca²⁺ and two separate Cl⁻ ions, each chloride with seven of its own outer electrons and one transferred electron. Show brackets and charges clearly, and check the total charge is zero. Do not put a shared pair between the ions: that would imply a covalent molecule.

Pearson 8CH0/01 June 2023 Q7(a) and its report identify swapped ion charges and covalent-style diagrams as common errors. The safe method is to determine each ion’s electron count and charge before drawing.

Swipe horizontally to view the whole diagram.

Compare ionic radii with electron count and nuclear charge

Down a group, comparable ions gain occupied shells and become larger. Ca²⁺ is larger than Mg²⁺ because it has one more occupied shell. A cation is generally smaller than its parent atom because removal of outer electrons reduces repulsion and can remove a whole shell. An anion is generally larger because added electrons increase repulsion while proton number stays the same.

Isoelectronic ions have the same number of electrons. N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ all contain ten. Their nuclear proton numbers increase from 7 to 13, so attraction to the same electron configuration increases and radius decreases in that order. ‘More positive charge means smaller’ is incomplete unless the electron counts being compared are understood.

Worked comparison: Ca²⁺, K⁺ and Cl⁻ have 18 electrons, but 20, 19 and 17 protons respectively. Therefore Ca²⁺ is smallest, followed by K⁺ and then Cl⁻. Br⁻ has an extra occupied shell and is larger than all three. The 2023 AS Q7(b) scheme uses precisely the isoelectronic and shell-count reasoning, rather than a vague periodic trend.

Ion charge and separation affect ionic attraction

For otherwise comparable lattices, larger ionic charges give stronger attractions, while a smaller distance between ion centres also strengthens attraction. MgO has Mg²⁺ and O²⁻ ions, whereas NaCl contains singly charged ions; the stronger attractions in MgO help explain its much higher melting temperature.

For two salts with the same anion and cations of equal charge, the smaller cation usually gives stronger ionic attraction. In explaining melting, identify attraction between ions, not between a nucleus and an outer electron. The latter describes ionisation energy. Actual melting temperatures also depend on lattice structure, so use the data and comparison supplied.

Electrical behaviour tests whether charged particles can move

Solid ionic compounds do not conduct because their ions are fixed in lattice positions. When molten, or dissolved in a suitable solvent, ions can move and carry charge. The solid already contains ions: melting releases their movement rather than creating them by splitting neutral molecules.

In an ion-migration demonstration, a coloured cation moves towards the negative electrode and a coloured anion towards the positive electrode under a direct-current field. Opposite migration directions support the existence of oppositely charged particles. A circuit needs a low-voltage supply, electrodes and an indicator of current; state the observation as well as the inference.

Use a teacher-approved microscale method, with suitable eye protection and controlled chemical disposal. Copper(II)/chromate examples require specific risk assessment because the compounds are hazardous; no high-voltage supply is needed. Compare movement with the field off to distinguish directed migration from ordinary diffusion.

Metallic bonding also involves electrostatic attraction

A metal is a giant lattice of positive metal ions surrounded by delocalised electrons. Metallic bonding is the strong electrostatic attraction between the positive ions and those electrons. Delocalised means the electrons are not confined to one bond or one atom; it does not mean electrons have disappeared.

Metals conduct as solids and liquids because electrons can move through the structure. Layers of ions can slide while attraction to the delocalised electrons persists, allowing malleability. In an ionic crystal, slipping layers can bring like charges next to each other; repulsion can fracture the crystal, explaining brittleness.

In 8CH0/01 June 2023 Q4(a), describing both the giant structure and the interacting particles was important. Naming a ‘sea of electrons’ alone does not identify the attraction or explain the properties.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources