Overview
What you’ll learn
- Describe the formation of ions by electron loss or gain, and ionic bonding between metals and non-metals with dot-and-cross diagrams.
- Relate the physical properties (including electrical property) of ionic compounds to their lattice structure.
- Describe covalent bonding as the sharing of electron pairs, using dot-and-cross diagrams for simple molecules.
- Deduce the arrangement of electrons in covalent molecules and relate the properties of covalent substances to their structure and bonding.
- Describe the differences between elements, compounds and mixtures.
- Describe the general physical properties of metals, describe alloys as mixtures, and identify metals and alloys from structure diagrams.
Key points
- 1Metal atoms lose their outer electrons to form positive ions; non-metals gain electrons to form negative ions — both reach noble-gas arrangements.
- 2An ionic bond is the electrostatic attraction between oppositely charged ions; dot-and-cross diagrams show outer shells only, with electrons from one atom as dots and from the other as crosses.
- 3Ionic compounds: high melting points (strong attractions), conduct only when molten or dissolved (mobile ions), and many dissolve in water.
- 4A covalent bond is a shared pair of electrons between two non-metal atoms; each atom counts the shared electrons in its outer shell.
- 5Covalent molecular substances: low melting and boiling points (weak forces between molecules overcome on melting — the covalent bonds never break) and they do not conduct.
- 6Elements are one substance only; compounds bond elements in fixed ratios; mixtures are just jumbled together and separable by physical means.
- 7Metals: shiny solids with high melting points, malleable and good conductors of heat and electricity.
- 8An alloy is a mixture of a metal with another element (brass = copper + zinc; stainless steel = iron + carbon + others); different-sized atoms distort the regular rows, making alloys harder than pure metals.
Notes · 1 of 6
Ionic bonding
Metals lose, non-metals gain — and the charged ions lock together.
A metal atom has few outer electrons; a non-metal has nearly a full outer shell. The efficient move is a transfer: the metal loses its outer electrons and the non-metal gains them, so both reach the electron arrangement of a noble gas. The metal becomes a positive ion (cation) and the non-metal a negative ion (anion).
- 1Write the electron arrangements
Sodium is 2.8.1; chlorine is 2.8.7.
- 2Move the electrons
Sodium's one outer electron transfers to chlorine: Na becomes Na⁺ (2.8) and Cl becomes Cl⁻ (2.8.8).
- 3Balance the charges
One Na⁺ (+1) balances one Cl⁻ (−1): formula NaCl. For Al³⁺ and O²⁻, two aluminiums (+6) balance three oxides (−6): Al₂O₃.
A dot-and-cross diagram shows only the outer shells: electrons from the metal are drawn as dots, those from the non-metal as crosses (or vice versa — the labels matter, not the symbols). Square brackets and the charge sit outside each ion: [Na]⁺ with an empty outer shell, [Cl]⁻ with eight electrons. Compounds like MgCl₂ need one magnesium for every two chlorines, because Mg gives one electron to each of two chlorine atoms.
Compounds like MgCl₂ show why charge balance beats picture-drawing: one magnesium gives one electron to each of two chlorine atoms, and the formula drops straight out of the arithmetic.
Notes · 2 of 6
Properties of ionic compounds
Strong attractions make them hard to melt; mobile ions make them conduct when liquid.
- High melting and boiling points: melting means pulling apart a giant lattice of ± attractions, and those attractions are strong.
- Conduct only when molten or dissolved: the ions are locked in place in the solid, but melting or dissolving frees them to move and carry charge.
- Often soluble in water (water molecules attract and surround both ions); the solution conducts for the same reason — mobile ions.
- Brittle as solids: shifting a layer brings like charges face to face and the lattice shatters.
Notes · 3 of 6
Covalent bonding
Two non-metals share pairs; every atom counts shared electrons as its own.
Two non-metal atoms both need electrons and neither will hand them over, so they share. A covalent bond is one shared pair of electrons; in a dot-and-cross diagram each atom contributes one electron to every pair. Counting rules: hydrogen is full with 2 electrons in its only shell; other non-metals aim for 8 (an octet).
| Molecule | Bonds | Counting check |
|---|---|---|
| H₂ | one shared pair | each H has 2 |
| O₂ | double bond — two shared pairs | each O counts 8 |
| H₂O | two single bonds on oxygen | O counts 8; each H has 2 |
| CH₄ | four single bonds on carbon | C counts 8; each H has 2 |
| CO₂ | two double bonds | C counts 8; each O counts 8 |
To deduce an unfamiliar molecule's arrangement, count what each atom still needs: nitrogen (2.8.5) needs three, so N₂ has a triple bond (three shared pairs); a hydrogen always offers exactly one. Shared pairs sit between the atoms; lone pairs sit on one atom only.
Notes · 4 of 6
Properties of covalent substances
Weak forces between molecules, strong bonds within them.
Covalent substances built from small molecules (H₂O, CO₂, CH₄, O₂) melt and boil easily: low melting and boiling points. The reason students must not mix up: melting separates molecules from each other, overcoming only the weak forces between molecules; the strong covalent bonds inside each molecule survive untouched. Water boiling is not water decomposing.
- No charged particles in any state — molecular liquids and solids do not conduct.
- Usually small molecules: gases or liquids at room temperature (the low melting/boiling points again).
- Mostly insoluble in water (but sugars dissolve because their −OH groups hydrogen-bond; keep the simple answer: small covalent molecules are generally insoluble).
Notes · 5 of 6
Elements, compounds and mixtures
One substance, bonded ratios, or a physical jumble.
| Type | What it is | Example | Separation |
|---|---|---|---|
| Element | one kind of atom only | oxygen gas, copper | not separable by chemical means |
| Compound | two or more elements chemically bonded in a fixed ratio | water (H₂O), sodium chloride | chemical change needed |
| Mixture | substances jumbled together, no fixed ratio | air, salt water, brass | physical separation (Chapter 1) |
Two fingerprints of a compound: its composition is fixed (water is always 2 H : 1 O), and it has new properties — sodium (a reactive metal) and chlorine (a toxic gas) become table salt. A mixture keeps its components' properties, and its composition is whatever you made it.
Notes · 6 of 6
Metals and alloys
Identical atoms in regular rows — and what happens when the rows are disturbed.
Metals share a set of general physical properties: they are shiny solids with high melting and boiling points, they conduct heat and electricity well, and they are malleable — hammerable into new shapes without shattering. In structure diagrams a metal appears as rows of identical atoms in a regular pattern.
An alloy is a mixture of a metal with another element: brass is copper with zinc; stainless steel is iron with carbon and other elements. In a structure diagram the alloy shows the regular rows interrupted by different-sized atoms. Those disturbances stop the layers sliding cleanly past each other, which is why alloys are harder and stronger than the pure metals they come from — the everyday reason bicycle frames and door hinges are alloys.
Revise
Key terms
The words examiners expect you to use precisely.
- ionic bond
- The strong electrostatic attraction between oppositely charged ions.
- cation
- A positive ion, formed when a metal atom loses electrons.
- anion
- A negative ion, formed when a non-metal atom gains electrons.
- giant lattice
- The regular, repeating three-dimensional arrangement of ions in an ionic compound.
- covalent bond
- A shared pair of electrons between two non-metal atoms.
- molecule
- A neutral group of atoms joined by covalent bonds, e.g. H₂O or CO₂.
Show 5 more key termsShow fewer key terms
- double bond
- Two shared pairs of electrons between the same two atoms, as in O₂ and CO₂.
- intermolecular forces
- The weak forces between separate molecules, overcome on melting and boiling — not the covalent bonds within molecules.
- compound
- Two or more elements chemically bonded together in a fixed ratio.
- alloy
- A mixture of a metal with other elements, e.g. brass (copper + zinc) or stainless steel.
- malleable
- Able to be hammered or pressed into new shapes without breaking — a general property of metals.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Writing a metal's ion with extra electrons.
Metals form cations by losing electrons; the ion is smaller in electrons, not larger.
Instead: Group 1 → 1+ charge (one lost); Group 2 → 2+ (two lost); Group 3 → 3+ (three lost).
Mistake: Writing NaCl₂ or similar formulae straight from the diagram.
Formulae come from charge balance, not from counting atoms in one diagram.
Instead: Total positive charge = total negative charge: Al³⁺ + O²⁻ → Al₂O₃ (swap-and-balance).
Show 9 more mistakesShow fewer mistakes
Mistake: Saying solid ionic compounds conduct because "electrons flow".
The charge carriers in ionic compounds are ions, and in the solid they are fixed.
Instead: Ionic compounds conduct only when molten or dissolved, when the ions themselves move.
Mistake: Saying water decomposes when it boils.
Boiling overcomes forces between molecules, not bonds inside them.
Instead: Steam is still H₂O — the covalent O–H bonds survive boiling.
Mistake: Giving hydrogen an octet in covalent diagrams.
Hydrogen's only shell holds two electrons — full at helium's duet.
Instead: H shares one pair and counts 2; the octet rule applies to the second-shell atoms.
Mistake: Drawing both atoms' electrons as the same symbol.
Dot-and-cross diagrams exist to show where each electron came from.
Instead: One atom contributes dots, the other crosses; shared pairs have one of each.
Mistake: Forgetting the square brackets and charges on ions in diagrams.
Without brackets, a lone chloride with eight electrons looks like an argon atom.
Instead: Ionic diagrams: [ion] with charge; covalent diagrams: no charges, no brackets.
Mistake: Calling brass a compound.
Brass has no fixed formula — the copper:zinc ratio is chosen by the maker.
Instead: Alloys are mixtures; compounds have fixed, bonded ratios.
Mistake: Saying alloys are chemically bonded new substances.
No electrons are transferred or shared between the different atoms in an alloy.
Instead: Alloying is a physical mixture; each metal keeps its own atoms.
Mistake: Classifying any high-melting substance as ionic.
Data classification needs more than one property.
Instead: Combine melting point with conductivity: ionic conducts when molten; metals conduct as solids.
Mistake: Explaining malleability with "weak bonds".
Metals have strong attractions — that is why they have high melting points.
Instead: Layers of identical atoms slide over each other without breaking the structure, so the shape changes but the metal stays whole.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Describe the formation of ions by electron loss or gain, and ionic bonding between metals and non-metals with dot-and-cross diagrams.
Which ion does a chlorine atom (2.8.7) form?
Ready for more practice on Chapter 3: Chemical Bonding and Structure?
Generate a fresh 5-question drill built from this chapter’s key points. Free with your study account.
Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 4Foundation
- 4Standard
- 4Challenging
12 questions worth 63 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsKeep practising
Practise this chapter
Jump straight to the questions for this chapter, the chapters either side of it, and this year’s assessment papers.
Test yourself
Full papers for this subject. Each school sets its own scope — check your assessment outline.
Exam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 3 Practice Paper
30 marks · 40 min · 4 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 3 (ions and ionic bonding with dot-and-cross diagrams, covalent molecules, properties from structure, elements/compounds/mixtures, metals and alloys).
Mark scheme
PROChapter 3 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
How do I decide between ionic and covalent bonding?
Metal + non-metal → ionic (electrons transferred). Non-metal + non-metal → covalent (electrons shared). Check the elements on the Periodic Table before drawing anything.
What belongs in a dot-and-cross diagram?
Outer shells only, with each atom's electrons drawn as a different symbol (dots vs crosses), shared pairs overlapping between atoms, and — for ions — square brackets with the charge outside. No inner shells needed.
Why do ionic compounds conduct only when molten or dissolved?
Conduction needs moving charged particles. In the solid the ions are locked in the lattice; melting or dissolving frees the ions themselves, and they carry the current — not electrons.
If covalent bonds are strong, why do molecular substances melt easily?
Because melting does not touch the covalent bonds. It only overcomes the weak forces between separate molecules — like separating marbles without scratching them.
What exactly is an alloy?
A mixture of a metal with another element (often another metal), e.g. brass (copper + zinc) or stainless steel (iron + carbon and others). The different-sized atoms disturb the regular rows, making alloys harder than pure metals.