What is an ion and how is it formed in the context of Cambridge IGCSE Coordinated Science?

An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge. In the context of Cambridge IGCSE Coordinated Science, ions are formed when atoms either lose electrons to become positively charged cations or gain electrons to become negatively charged anions. This process typically occurs during chemical reactions where atoms seek to achieve a stable electron configuration, often resembling that of the nearest noble gas.

How do ionic bonds form between atoms in elements and compounds?

Ionic bonds form when atoms transfer electrons to achieve stable electron configurations. This typically occurs between metals and non-metals. Metals, which have few electrons in their outer shell, lose electrons to become positively charged cations. Non-metals, which have more electrons in their outer shell, gain these electrons to become negatively charged anions. The opposite charges of the ions attract each other, resulting in a strong electrostatic force known as an ionic bond, which holds the ions together in a compound.

What are some common properties of ionic compounds studied in IGCSE Coordinated Science?

Ionic compounds, which are a key focus in IGCSE Coordinated Science, typically have high melting and boiling points due to the strong electrostatic forces between the ions. They are generally solid at room temperature and can conduct electricity when melted or dissolved in water, as the ions are free to move. Additionally, ionic compounds are often soluble in water but insoluble in non-polar solvents.

Why do atoms form ions, and how does this relate to the periodic table?

Atoms form ions to achieve a stable electron configuration, often similar to that of the nearest noble gas, which is a key concept in the study of elements and compounds in IGCSE Coordinated Science. The periodic table helps predict ion formation as elements in the same group typically form ions with the same charge. For example, Group 1 elements lose one electron to form +1 cations, while Group 17 elements gain one electron to form -1 anions.

Can you explain the difference between ionic and covalent bonds?

In IGCSE Coordinated Science, understanding the difference between ionic and covalent bonds is crucial. Ionic bonds involve the transfer of electrons from one atom to another, resulting in the formation of charged ions. These ions are held together by electrostatic forces. In contrast, covalent bonds involve the sharing of electron pairs between atoms, typically non-metals, to achieve stability. While ionic bonds form between metals and non-metals, covalent bonds usually occur between non-metal atoms.

Why is "Saying ionic bonds involve sharing electrons" a common mistake in Ions and Ionic Bonds?

Why it happens: Students confuse ionic bonding (electron transfer) with covalent bonding (electron sharing). How to avoid it: Ionic = transfer; covalent = share. Use 'transfer' in ionic questions and 'sharing' in covalent questions. Source: 0654 Examiner Report 2022

Why is "Saying ionic compounds conduct electricity when solid" a common mistake in Ions and Ionic Bonds?

Why it happens: Students know ionic compounds have ions but forget they must be free to move. How to avoid it: Solid: ions fixed in lattice — no conduction. Molten or dissolved: ions free to move — conduction.

Why is "Forgetting to write ionic charges on the dot-and-cross diagram" a common mistake in Ions and Ionic Bonds?

Why it happens: Students draw the electron configurations correctly but omit the charge labels. How to avoid it: Always put square brackets around each ion and write the charge (e.g. Na⁺) outside the bracket. Source: 0654 Examiner Report 2023

Atoms, Elements and CompoundsCambridge IGCSE Coordinated Sciences 0654

Ions and Ionic Bonds

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Ionic Bonding — Cambridge IGCSE Coordinated Science 0654

Ionic bonds form when electrons transfer from metals to non-metals, creating oppositely charged ions held by electrostatic attraction. Cambridge tests dot-and-cross diagrams, ionic properties, and formulae from ion charges.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C3.8 — Describe ionic bonding in terms of electron transfer.
C3.9 — Draw dot-and-cross diagrams for ionic compounds.
C3.10 — Explain the properties of ionic compounds.
C3.11 — Deduce the formula of ionic compounds from ion charges.

Formation of Ionic Bonds

Ionic bonds form when a metal atom transfers electrons to a non-metal atom, forming oppositely charged ions that attract each other.

Ionic bonding: the electrostatic force of attraction between oppositely charged ions formed by the transfer of electrons from a metal to a non-metal.

Formation of NaCl:

Sodium: 2,8,1 → loses 1 electron → Na⁺ (2,8) — noble gas configuration of Ne
Chlorine: 2,8,7 → gains 1 electron → Cl⁻ (2,8,8) — noble gas configuration of Ar
Na⁺ and Cl⁻ attract each other → ionic bond

Formation of MgO:

Magnesium: 2,8,2 → loses 2 electrons → Mg²⁺ (2,8)
Oxygen: 2,6 → gains 2 electrons → O²⁻ (2,8)

Dot-and-cross diagrams for ionic compounds:

Show outer shell electrons only
Transfer is shown by dots and crosses from different atoms
Square brackets around each ion with charge outside

Deducing ionic formulae:

Balance charges so the compound is electrically neutral
Mg²⁺ + 2Cl⁻ → MgCl₂ (2 chloride ions balance one magnesium ion)
Al³⁺ + 3OH⁻ → Al(OH)₃

Metal loses electrons (cation); non-metal gains electrons (anion). Both achieve noble gas config.
Ionic bond = electrostatic attraction between oppositely charged ions.
Formula: balance ion charges to make neutral compound (Mg²⁺ + 2Cl⁻ → MgCl₂).

Properties of Ionic Compounds

High melting points, conduct when dissolved/molten, brittle — all explained by the giant ionic lattice structure.

Giant ionic lattice: Ionic compounds form a regular 3D lattice of alternating positive and negative ions. Millions of electrostatic attractions in all directions.

Properties and explanations:

Property	Explanation
High melting/boiling point	Many strong electrostatic forces between ions require a lot of energy to break
Conduct electricity when molten	Ions are FREE to move → carry charge
Conduct electricity when dissolved in water	Ions separate (dissociate) → free to move
Do NOT conduct when solid	Ions in fixed positions → cannot move to carry charge
Brittle	If layers are displaced, like charges align → strong repulsion → crystal shatters
Soluble in water (usually)	Water molecules attract and surround ions (hydration)
Insoluble in organic solvents	Organic solvents cannot break up the strong electrostatic forces

High MP/BP: many strong electrostatic forces to break.
Conducts when molten or dissolved (free ions). Does NOT conduct in solid state.
Brittle: displaced layers → like charges repel → shatters.

How it’s examined

Paper 4: Draw the dot-and-cross diagram for MgO or NaCl (3 marks — transfer shown, correct charges, correct configurations). 'Explain why sodium chloride has a high melting point' (2 marks — giant ionic lattice, many strong electrostatic forces). 'Explain why solid NaCl does not conduct electricity but aqueous NaCl does' (2 marks — solid: ions fixed; aqueous: ions free to move). Formulae from charges are tested in Paper 2 MCQ.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — Ions and Ionic Bonds

Step-by-step solutions to past-paper-style questions on ions and ionic bonds, written exactly the way a tutor would explain them at the board.

1Dot-and-cross diagram of sodium chloride
Extended
Question
Draw a dot-and-cross diagram to show the ionic bonding in sodium chloride (NaCl). Indicate the charges on the ions formed.
Step-by-step solution
1. Step 1
  Na has electronic configuration 2,8,1. It loses its single outer electron to form Na⁺ with configuration 2,8 (noble-gas configuration of Ne).
2. Step 2
  Cl has electronic configuration 2,8,7. It gains one electron to form Cl⁻ with configuration 2,8,8 (noble-gas configuration of Ar).
3. Step 3
  Show Na⁺ with a full outer shell of 8 electrons (using crosses from Na) and Cl⁻ with a full outer shell of 8 electrons (7 dots from Cl + 1 cross from Na). Label charges as Na⁺ and Cl⁻.
Answer
Na⁺ (2,8 configuration) and Cl⁻ (2,8,8 configuration) held together by electrostatic attraction; Na⁺ has a full diagram with [He] core + 8-electron outer shell (crosses); Cl⁻ has 8-electron outer shell (7 dots + 1 cross).
Examiner tip
Show electrons from Na as crosses and electrons from Cl as dots (or vice versa). Charge labels are essential. Square brackets around each ion with the charge outside are required.
2Why ionic compounds have high melting points
Extended
Question
Explain why ionic compounds such as sodium chloride have high melting points.
Step-by-step solution
1. Step 1
  In the solid state, ionic compounds form a giant ionic lattice — a regular three-dimensional arrangement of oppositely charged ions.
2. Step 2
  Strong electrostatic forces of attraction act between each positive ion and surrounding negative ions in all directions throughout the lattice.
3. Step 3
  A large amount of energy is needed to overcome these strong attractions and separate the ions; this requires a high temperature, giving a high melting point.
Answer
Strong electrostatic attraction between oppositely charged ions throughout the giant ionic lattice requires a large amount of energy to overcome; hence a high melting point.
Examiner tip
Key marking phrases: 'giant ionic lattice', 'strong electrostatic attraction', 'large amount of energy to overcome'.
3Electrical conductivity — solid vs molten NaCl
Extended
Question
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
Step-by-step solution
1. Step 1
  In solid NaCl, Na⁺ and Cl⁻ ions are held in fixed positions in the ionic lattice; they cannot move to carry electric charge.
2. Step 2
  When NaCl is melted, the lattice breaks down; the ions become free to move through the melt.
3. Step 3
  Free-moving ions can carry charge to the electrodes, so molten NaCl conducts electricity.
Answer
Solid: ions fixed in lattice, cannot move, no conduction. Molten: ions free to move, can carry charge, conducts electricity.
Examiner tip
The word 'free' is important in the mark scheme: 'free to move' or 'mobile ions'.
4MgO vs NaCl melting points
Extended
Question
Magnesium oxide (MgO) has a melting point of $2852\,°\text{C}$ , much higher than sodium chloride ( $801\,°\text{C}$ ). Suggest why.
Step-by-step solution
1. Step 1
  MgO contains Mg²⁺ and O²⁻ ions; NaCl contains Na⁺ and Cl⁻ ions.
2. Step 2
  The charges on Mg²⁺ and O²⁻ (+2 and −2) are greater than those on Na⁺ and Cl⁻ (+1 and −1).
3. Step 3
  Greater ionic charges → stronger electrostatic attraction between ions → more energy required to separate them → higher melting point.
Answer
Mg²⁺/O²⁻ have higher charges (+2, −2) than Na⁺/Cl⁻ (+1, −1); stronger electrostatic attraction; more energy to overcome → higher melting point.

Key Definitions and Keywords — Ions and Ionic Bonds

Definitions to memorise and the exact keywords mark schemes credit for ions and ionic bonds answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Ionic bond
Examiner keyword
The strong electrostatic attraction between oppositely charged ions formed by the transfer of one or more electrons from a metal atom to a non-metal atom.
Cation
Examiner keyword
A positively charged ion formed when an atom loses one or more electrons (typically a metal).
Anion
Examiner keyword
A negatively charged ion formed when an atom gains one or more electrons (typically a non-metal).
Giant ionic lattice
Examiner keyword
A regular, three-dimensional arrangement of alternating positive and negative ions held together by strong electrostatic attractions in all directions.
Electrostatic attraction
Examiner keyword
The force of attraction between oppositely charged particles (e.g. positive and negative ions); the basis of ionic bonding.

Common Mistakes and Misconceptions — Ions and Ionic Bonds

The traps other students keep falling into on ions and ionic bonds questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Saying ionic bonds involve sharing electrons
0654 Examiner Report 2022
Why it happens
Students confuse ionic bonding (electron transfer) with covalent bonding (electron sharing).
How to avoid it
Ionic = transfer; covalent = share. Use 'transfer' in ionic questions and 'sharing' in covalent questions.
✕Saying ionic compounds conduct electricity when solid
Why it happens
Students know ionic compounds have ions but forget they must be free to move.
How to avoid it
Solid: ions fixed in lattice — no conduction. Molten or dissolved: ions free to move — conduction.
✕Forgetting to write ionic charges on the dot-and-cross diagram
0654 Examiner Report 2023
Why it happens
Students draw the electron configurations correctly but omit the charge labels.
How to avoid it
Always put square brackets around each ion and write the charge (e.g. Na⁺) outside the bracket.

Practice questions

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