Ions form by electron loss or gain; charge follows the group
An ion forms when an atom loses or gains electrons. Group 1, 2 and 3 metals lose outer electrons to give positive ions of charge , , ; Group 5, 6 and 7 non-metals gain electrons to give negative ions of charge , , (charge = group number ). Each ion gains a full outer shell. Named ions (, , , , ) are not deducible from the group and must be learnt.
Ionic bond: electrostatic attraction between oppositely charged ions in a giant lattice
An ionic bond is the strong electrostatic attraction between oppositely charged ions — not a shared pair of electrons (that is covalent). Ions pack into a giant ionic lattice: a regular, repeating 3-D arrangement with strong forces acting in all directions. The formula gives the simplest whole-number ratio of ions (e.g. is one per ), not the atoms of a molecule — ionic compounds are not separate molecules.
Giant lattice: explains high melting points and the conductivity rules
Ionic compounds have high melting and boiling points: the many strong electrostatic forces between the oppositely charged ions need much energy to overcome. Conductivity depends on whether ions are free to move — a solid does not conduct (ions fixed in the lattice), but molten and aqueous conduct because the lattice breaks up and ions become mobile. Link each property to structure: forces melting point; free ions conducts.
Drawn from real examiner reports.
Say "giant lattice", not just "giant"
Describing an ionic structure as merely giant does not score — the mark scheme wants giant (ionic) lattice. The word lattice signals the regular, repeating 3-D arrangement of ions held by electrostatic forces acting in all directions. Loose answers like "giant structure" or "giant molecule" also miss the mark; ionic compounds are lattices of ions, never molecules.
Giant ionic structures called merely "giant" rather than "giant lattice" lost the mark (Jun23 Q3a).
Ionic formulae that don't charge-balance
When writing a formula the total positive charge must exactly equal the total negative charge. A ion needs two ions (); a with a crosses over to . Writing "MgCl" or "AlO" (charges unbalanced) is a frequent error. A polyatomic ion used more than once goes in brackets: , not .
Ionic bond ≠ sharing/transferring electrons
An ionic bond is not atoms sharing electrons (that is covalent) and not simply the transfer of electrons (that is how the ions form, not the bond). The definition needs both parts: electrostatic attraction between oppositely charged ions. Answers that drop the word electrostatic, or leave out that the particles are ions, are not credited.
Bond described as electron sharing or transfer, or the word electrostatic omitted — definition not credited (Jun23 Q5c).
Dot-and-cross: missing charges or shells
In dot-and-cross diagrams for ionic compounds, marks are lost for omitting the overall charge on each ion (shown as a superscript with the ion in square brackets), for not showing inner shells where required, and for copying two atoms with no electron transfer drawn. After transfer a metal ion has an empty outer shell; the non-metal a full shell of eight.
Dot-and-cross marks lost for missing or wrong charges, absent inner shells, or no electron transfer shown (Jun24 1CR Q9bii).
Ionic solids do not conduct electricity
A solid ionic compound does not conduct — its ions are locked in fixed positions in the lattice. When explaining why the molten or dissolved compound conducts, "it has ions" or "the ions carry charge" is not enough: the key idea is that the ions are now free to move. Conduction needs charged ions that are free to move; in the solid the ions exist but cannot move.
Charge carriers are ions, not electrons
In an ionic compound the mobile charge carriers are the ions themselves, not free or delocalised electrons (those carry charge in metals and in graphite). When molten or in solution it is the free-moving positive and negative ions that carry the current. Naming electrons as the carriers in an ionic compound loses the mark.
MgCl2 is ionic, not covalent
When comparing melting points, candidates wrongly treat a metal + non-metal compound such as as covalent. A metal with a non-metal is ionic (giant lattice, high melting point); only non-metal + non-metal is simple molecular. The related error is describing "breaking covalent bonds" in a molecular substance, not overcoming its weak intermolecular forces.
MgCl2 treated as covalent when comparing giant-ionic and simple-molecular melting points (Jun24 1CR Q9biii).
Use the cross-over method for formulae
Write each ion with its charge (metal = group number; non-metal = group number ). Cross the charges over to subscripts, simplify to the smallest ratio (), bracket any polyatomic ion used twice, then check charges balance.
Link every property back to the structure
Explanations need linked statements, not a bare fact. Melting point: giant lattice strong electrostatic forces large energy to overcome high melting point. Conductivity: say whether ions are free to move, then that they carry the charge.
Dot-and-cross: bracket and charge every ion
Show only the outer shells. Draw the transfer from metal to non-metal (dots for one, crosses for the other). Put every ion in square brackets with its overall charge — the most-forgotten mark. The metal ion ends with an empty outer shell, the non-metal a full shell of eight.
Charges you can read from the group:
| Group | 1 | 2 | 3 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|
| Ion charge | ||||||
| Example |
Named ions to learn (their charge is not obvious from the table):
| , , | , , , | , | , |
Define an ion.
Magnesium is in Group 2 and oxygen is in Group 6. Deduce the charge on each ion and hence write the formula of magnesium oxide. (3 marks)
