Ions and Ionic Bonding
Ionic bonding happens between a metal and a non-metal, by transferring electrons. The physics is simple; the marks depend almost entirely on using the right phrase.
1. The definition
An ionic bond is the strong ELECTROSTATIC FORCE OF ATTRACTION between OPPOSITELY CHARGED IONS.
This is worth 2 marks, and both halves are needed. Tutors flagged the exact wording: “electrostatic force of attraction” and “oppositely charged ions”. An incomplete definition was a recorded error.
A bond is a FORCE OF ATTRACTION, not a line. One student described atoms as “stuck together by a line” — the line on a diagram represents a force.
Never say the ions are attracted “because of protons”. It is the overall charges on the ions that attract — a recorded error mentioned protons instead of electrons.
2. How ions form
Atoms bond to achieve a FULL OUTER SHELL — the stable electronic configuration of a noble gas.
METALS LOSE electrons → positive ions (cations) NON-METALS GAIN electrons → negative ions (anions)
Metals lose to non-metals — never the reverse. A recorded error had non-metals transferring electrons to metals. Metals have few outer electrons and lose them easily; non-metals have nearly full shells and gain.
Worked example — sodium chloride:
- Sodium (2,8,1) loses its 1 outer electron → Na⁺ with configuration 2,8
- Chlorine (2,8,7) gains 1 electron → Cl⁻ with configuration 2,8,8
- The oppositely charged ions attract → NaCl
Worked example — magnesium oxide:
- Mg (2,8,2) loses 2 → Mg²⁺ (2,8)
- O (2,6) gains 2 → O²⁻ (2,8)
- → MgO
Worked example — lithium bromide:
- Lithium (the metal) donates; bromine (the non-metal) accepts
Identify which element donates before you start. Confusion over which element donates in lithium bromide was recorded — it is always the metal.
Predicting the charge from the group:
| Group | Outer electrons | Ion formed |
|---|---|---|
| I | 1 | 1+ |
| II | 2 | 2+ |
| III | 3 | 3+ |
| V | 5 | 3− |
| VI | 6 | 2− |
| VII | 7 | 1− |
Elements with two outer electrons LOSE them — they don’t gain six. A recorded error assumed the opposite. Losing 2 is far easier than gaining 6.
Group IV elements generally do not form simple ions — losing or gaining four electrons costs too much energy, so they bond covalently. A recorded claim that Group IV “does not form compounds” is wrong — carbon and silicon form vast numbers of covalent compounds.
3. Dot-and-cross diagrams
Conventions:
- Use dots for one atom’s electrons and crosses for the other’s, so the transfer is visible
- Show square brackets around each ion
- Write the charge outside the top right of the brackets
- For ionic compounds, show only the outer shell unless asked for all shells
For NaCl: [Na]⁺ with an empty outer shell, and [Cl with 8 outer electrons]⁻ — one of which is a dot to show it came from sodium.
Mention the specific charges to gain full marks — tutors flagged this. Brackets without charges lose the mark.
Check the electron count after transfer. A recorded error stated chlorine has “17 electrons” after gaining one — it has 17 protons and, as Cl⁻, 18 electrons.
For compounds with a 2:1 ratio (e.g. MgCl₂), draw two chloride ions.
4. The giant ionic lattice
Ionic compounds form a GIANT IONIC LATTICE — a regular, repeating 3D arrangement of oppositely charged ions, held together by strong electrostatic forces in all directions.
A lattice is a regular repeating 3D structure — worth knowing as a one-mark definition, which tutors flagged.
It is not a molecule. There are no individual “NaCl units” — the formula gives the ratio of ions in the lattice.
5. Explaining the properties
Every property follows from the lattice and the strong electrostatic forces. This is the most examined part of the topic.
High melting and boiling points
There are strong electrostatic forces of attraction between the oppositely charged ions throughout the lattice, which require a lot of energy to overcome.
Say “electrostatic forces of attraction” explicitly — tutors flagged this as the phrase that earns the mark.
The high melting point is NOT “because they contain metals”. This exact error was recorded. It is because of the strong forces in the lattice.
Electrical conductivity
This is where most marks are lost, and the rule is precise:
| State | Conducts? | Why |
|---|---|---|
| Solid | NO | ions are fixed in position and cannot move |
| Molten (liquid) | YES | ions are free to move and carry charge |
| Aqueous (dissolved) | YES | ions are free to move |
The charge carriers are IONS, not electrons. A recorded error referred to electrons — in an ionic compound it is the mobile ions that conduct. (Electrons carry the current in metals.)
Say “the ions are free to move”. Understanding that ions are fixed in the solid but mobile when molten or aqueous was a recorded difficulty, and it is the whole explanation.
Solubility and brittleness
Most ionic compounds are soluble in water and insoluble in organic solvents.
They are brittle: a blow shifts the layers so that like charges align, and they repel, splitting the crystal.
Don’t confuse solubility with conductivity. Recorded as an error — they are separate properties with separate explanations.
6. Ionic vs covalent
| Ionic | Covalent | |
|---|---|---|
| Between | metal + non-metal | non-metal + non-metal |
| Electrons are | TRANSFERRED | SHARED |
| Particles | ions | molecules (or giant structures) |
Ionic = transfer. Covalent = share. Confusing sharing with transfer was recorded four separate times — it is the single most common error in bonding. If a metal is involved, electrons are transferred.
Intramolecular vs intermolecular: the ionic or covalent bond is the force within the compound; intermolecular forces act between separate molecules and are much weaker. Confusing them was recorded.
7. Mistakes that cost marks
Saying electrons are shared in an ionic bond.
An incomplete definition — missing “electrostatic” or “oppositely charged ions”.
Describing a bond as a line.
Saying non-metals give electrons to metals.
Giving aluminium a negative charge.
Saying Group II elements gain six electrons.
Miscounting electrons after transfer.
Omitting brackets or charges on a dot-and-cross diagram.
Explaining high melting points by “containing metals”.
Saying solid ionic compounds conduct.
Saying electrons carry the charge in molten ionic compounds.
Confusing intramolecular with intermolecular forces.
Frequently asked questions
What is an ionic bond? The strong electrostatic force of attraction between oppositely charged ions.
Which elements form ionic bonds? A metal and a non-metal.
What happens to the electrons? They are transferred from the metal to the non-metal.
Why do atoms form ions? To achieve a full outer shell, like a noble gas.
What charge does a Group II element form? 2+ — it loses its two outer electrons.
What is a giant ionic lattice? A regular repeating 3D arrangement of oppositely charged ions.
Why do ionic compounds have high melting points? Strong electrostatic forces between ions throughout the lattice need a lot of energy to overcome.
Do ionic compounds conduct electricity? Not when solid (ions fixed), but yes when molten or aqueous (ions free to move).
What carries the charge? Mobile ions — not electrons.
Why are ionic compounds brittle? A blow aligns like charges, which repel and split the crystal.
Quick revision checklist
- I can define an ionic bond using both key phrases
- I know ionic bonds form between a metal and a non-metal
- I know electrons are transferred, not shared
- I know metals lose and non-metals gain
- I can predict ion charges from the group number
- I can draw dot-and-cross diagrams with brackets and charges
- I can check the electron count after transfer
- I can define a giant ionic lattice
- I can explain high melting points with electrostatic forces
- I know the conductivity rule for solid, molten and aqueous
- I know ions carry the charge, not electrons
- I can explain brittleness
- I can distinguish ionic from covalent, and intra- from intermolecular
These notes cover ions and ionic bonding in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and data booklet for your own exam series.
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