What are ions and how are they formed in Chemistry?

Ions are charged particles that form when atoms gain or lose electrons. In Chemistry, particularly at the Cambridge IGCSE level, ions are crucial for understanding chemical reactions. When an atom loses electrons, it becomes a positively charged ion, known as a cation. Conversely, when an atom gains electrons, it becomes a negatively charged ion, called an anion.

How do ionic bonds form between elements?

Ionic bonds form when there is a transfer of electrons from one atom to another, resulting in the formation of ions. This typically occurs between metals and non-metals. The metal atom loses electrons to become a cation, while the non-metal atom gains those electrons to become an anion. The opposite charges of the ions attract each other, creating a strong ionic bond.

What are some common properties of ionic compounds?

Ionic compounds, which are formed through ionic bonds, have several distinctive properties. They generally have high melting and boiling points due to the strong attraction between ions. Ionic compounds are also typically soluble in water and conduct electricity when dissolved or molten, as the ions are free to move and carry charge.

Why are ionic bonds important in the study of Chemistry?

Ionic bonds are fundamental to understanding how elements combine to form compounds. They explain the formation of many common substances, such as table salt (sodium chloride). In the Cambridge IGCSE Chemistry curriculum, ionic bonds are essential for grasping concepts related to chemical reactions, compound formation, and the properties of materials.

How can you determine the formula of an ionic compound?

To determine the formula of an ionic compound, you need to balance the total positive and negative charges of the ions involved. For example, in sodium chloride, the sodium ion (Na+) and the chloride ion (Cl-) combine in a 1:1 ratio to balance the charges, resulting in the formula NaCl. Understanding the charges of common ions is key to writing correct formulas for ionic compounds.

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

Why it happens: Confusing ionic bonding with covalent bonding. How to avoid it: Ionic = TRANSFER of electrons (metal → non-metal), forming charged ions. Covalent = SHARING of electrons between non-metals. Source: 0620/42 — recurring

Why is "Writing $\text{MgCl}$ for magnesium chloride." a common mistake in Ions and Ionic Bonds?

Why it happens: Forgetting that the charges must balance. How to avoid it: Total positive charge must equal total negative charge. Mg^2+ (+2) needs TWO Cl^- (-1 each) → MgCl_2.

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

Why it happens: Knowing ionic compounds 'conduct' without linking it to mobile ions. How to avoid it: Conduction needs MOBILE charge carriers. In a solid the ions are FIXED in the lattice; only molten or aqueous ionic compounds (free-moving ions) conduct.

Why is "Saying ionic compounds conduct because electrons are free to move." a common mistake in Ions and Ionic Bonds?

Why it happens: Mixing up ionic conduction with metallic conduction. How to avoid it: In ionic compounds it is the IONS that move and carry charge, not electrons. Delocalised electrons carry charge in metals, not ionic substances.

Cambridge IGCSE Chemistry (0620)

Ions and Ionic Bonds

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Short Notes - Ions and Ionic Bonds

Detailed Study Notes

Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.

Ions and Ionic Bonds — Cambridge IGCSE 0620 Chemistry Extended (2026)

Atoms gaining/losing electrons to reach a noble-gas configuration; the giant ionic lattice; properties (high melting points, conduct when molten/dissolved).

At a glance

Ion: atom or group of atoms with a charge (lost or gained electrons).
Cation ( $+$ ): metal lost electrons. Anion ( $-$ ): non-metal gained electrons.
Ionic bond: electrostatic attraction between oppositely charged ions.
Forms a giant ionic LATTICE — billions of ions in a 3D pattern.
Properties: high m.p./b.p., crystalline, conduct when MOLTEN or DISSOLVED, NOT solid.
Group 1 → +1; Group 2 → +2; Group 17 → −1; Group 16 → −2.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

2.1 — Describe the formation of ions by electron loss/gain.
2.1 — Describe ionic bonding and the formation of an ionic lattice.
2.1 — Predict the charges of ions for elements in groups 1, 2, 16, 17.
2.1 — Describe physical properties of ionic compounds.

How ions form

Atoms lose or gain electrons to reach a full outer shell (noble-gas configuration).

The driving force. Atoms are most stable when they have a FULL OUTER SHELL of electrons (a noble-gas configuration). They lose, gain, or share electrons to reach that state.

Metals lose electrons to become positive cations.

Sodium ( $2, 8, 1$ ) loses 1 e⁻ → Na⁺ ( $2, 8$ , like neon).
Magnesium ( $2, 8, 2$ ) loses 2 e⁻ → Mg²⁺ ( $2, 8$ ).
Aluminium ( $2, 8, 3$ ) loses 3 e⁻ → Al³⁺ ( $2, 8$ ).

Non-metals gain electrons to become negative anions.

Chlorine ( $2, 8, 7$ ) gains 1 e⁻ → Cl⁻ ( $2, 8, 8$ , like argon).
Oxygen ( $2, 6$ ) gains 2 e⁻ → O²⁻ ( $2, 8$ , like neon).
Nitrogen ( $2, 5$ ) gains 3 e⁻ → N³⁻ ( $2, 8$ ).

Group rules.

Group	Charge formed
1	$+1$
2	$+2$
13	$+3$
15	$-3$
16	$-2$
17	$-1$
18	already full — usually no ions

Worked. Sulfur ( $2, 8, 6$ ) → S²⁻ (gain 2 to fill outer shell).

Metals: lose electrons → +ve cations.
Non-metals: gain electrons → -ve anions.
Goal: full outer shell (noble gas configuration).
Group rules give predicted charges.

Ionic bonding and the lattice

Opposite ions attract by electrostatic force; build up a 3D giant lattice.

Ionic bond. The strong electrostatic ATTRACTION between oppositely charged ions.

Forming sodium chloride.

Na atom loses 1 e⁻ → Na⁺.
Cl atom gains that e⁻ → Cl⁻.
Na⁺ and Cl⁻ attract each other electrostatically.
Many Na⁺ and Cl⁻ ions arrange themselves in a 3D pattern — a giant ionic lattice — alternating + and − ions.

Sodium gives its single outer electron to chlorine; both reach a full outer shell and the resulting ions attract.

Drawing dot-and-cross diagrams. Show outer-shell electrons only.

For NaCl, you'd draw Na surrendering its outer × to Cl, then Cl shown with 8 dots (full shell). The two ions are drawn next to each other in square brackets, with their charges.

Stoichiometry from charges. Total positive charge = total negative charge in the formula.

Na⁺ and Cl⁻ → NaCl (one of each).
Mg²⁺ and Cl⁻ → MgCl₂ (one Mg²⁺ balances two Cl⁻).
Al³⁺ and O²⁻ → Al₂O₃ (lowest common multiple is 6: 2×3 = 3×2).

Worked. Calcium reacting with bromine.

Ca: $2, 8, 8, 2$ → Ca²⁺.
Br: gains 1 e⁻ → Br⁻.
Need 2 Br⁻ to balance one Ca²⁺ → CaBr₂.

Two 1− bromide ions are needed to cancel one 2+ calcium ion, so the formula is CaBr₂.

Ionic bond: opposite ions attract electrostatically.
Build up: giant 3D ionic lattice.
Total $+$ = total $-$ in formula.
Mg²⁺ + 2 Cl⁻ → MgCl₂.

Properties of ionic compounds

High m.p., crystalline, soluble in water, conduct only when molten/dissolved.

1. High melting and boiling points. Strong electrostatic forces in the lattice need lots of energy to break. NaCl m.p. = $801\degree\text{C}$ .

2. Crystalline. Regular lattice → regular shape → crystals.

3. Often soluble in water. Polar water molecules pull ions out of the lattice.

4. Electrical conductivity.

Solid: NO — ions can't move (locked in the lattice).
Molten: YES — heating breaks the lattice; ions free to move and carry charge.
Dissolved: YES — water-separated ions carry charge.

Property	Ionic compound	Reason
Melting / boiling point	High (NaCl 801 °C)	Strong electrostatic forces throughout the lattice
Structure	Crystalline	Regular repeating lattice gives a regular shape
Solubility in water	Often soluble	Polar water molecules pull ions out of the lattice
Conducts as a solid	No	Ions are locked in the lattice and cannot move
Conducts when molten	Yes	Lattice is broken, so ions are free to move
Conducts when dissolved	Yes	Ions are separated by water and free to move

In the lattice every ion is locked in place — that is why a solid ionic compound does not conduct.

5. Brittle. Force in one direction shifts ions; like ions repel; lattice cracks along the cleavage plane.

Cambridge tip. When asked "why does sodium chloride conduct when molten but not when solid?" — the answer is about ION MOBILITY, not about the existence of ions. Ions exist in both states; they just can't move in the solid.

Worked. Why is the m.p. of MgO ( $2852\degree\text{C}$ ) higher than NaCl ( $801\degree\text{C}$ )? Both are ionic. MgO has $2+$ and $2-$ ions — the electrostatic attraction is stronger (charges 2 vs 1) → harder to break the lattice → higher m.p.

High m.p. and b.p.
Crystalline.
Often water-soluble.
Conducts molten/dissolved, NOT solid.
Brittle.

Quick recap

Metal: lose e⁻ → cation. Non-metal: gain e⁻ → anion.
Ionic bond: electrostatic attraction.
Giant ionic lattice: 3D alternating + and −.
Stoichiometry from charge balance.
High m.p., crystalline, conduct molten/dissolved only.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Ion — atom or group of atoms with a net electrical charge.
Cation — positively charged ion (formed by losing electrons).
Anion — negatively charged ion (formed by gaining electrons).
Ionic bond — strong electrostatic attraction between oppositely charged ions.
Giant ionic lattice — 3D regular pattern of alternating cations and anions.

How it’s examined

Ionic bonding is examined every Paper 2 (4-5 marks: predict ion charges, balance a formula) and every Paper 4 (6-8 marks: dot-and-cross diagrams, properties, conductivity). Examiner reports flag wrong charges (especially for transition metals — they often have variable charges; Cambridge usually quotes the charge in the question).

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (2.1); 0620/42 Oct/Nov 2024 — Q5 (ionic bonding); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

Take this whole topic with you

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.

1Formation of $\text{NaCl}$ from atoms
Getting started• NaCl, electron transfer
Question
Show, using electron configurations, how a sodium atom and a chlorine atom form sodium chloride.
Step-by-step solution
1. Step 1
  Sodium ( $2,8,1$ ) has one electron in its outer shell. A metal LOSES electrons, so it loses this one electron to form $\text{Na}^{+}$ with the configuration $2,8$ .
  $\text{Na} \to \text{Na}^{+} + e^{-}$
2. Step 2
  Chlorine ( $2,8,7$ ) needs one more electron to fill its outer shell. A non-metal GAINS electrons, so it gains the electron lost by sodium to form $\text{Cl}^{-}$ with the configuration $2,8,8$ .
  $\text{Cl} + e^{-} \to \text{Cl}^{-}$
3. Step 3
  Both ions now have a full outer shell (noble-gas configuration). The oppositely charged ions are held together by strong electrostatic attraction in a giant ionic lattice.
  $\text{Na}^{+} + \text{Cl}^{-} \to \text{NaCl}$
Answer
$\text{Na} \to \text{Na}^{+} + e^{-}$ (loses 1e, becomes $2,8$ ); $\text{Cl} + e^{-} \to \text{Cl}^{-}$ (gains 1e, becomes $2,8,8$ ); the $\text{Na}^{+}$ and $\text{Cl}^{-}$ ions attract electrostatically to give $\text{NaCl}$ .
Examiner tip
One electron is transferred FROM the metal TO the non-metal. Always state the final ion charges and that the attraction is electrostatic.
2Predicting ion charges from group number
Getting started• ion charges, group number
Question
Predict the charge on the ion formed by each of these elements: potassium (Group I), calcium (Group II), aluminium (Group III), oxygen (Group VI) and bromine (Group VII).
Step-by-step solution
1. Step 1
  Metals in Groups I, II and III LOSE their outer electrons to reach a full shell. The charge equals the group number: K $\to \text{K}^{+}$ , Ca $\to \text{Ca}^{2+}$ , Al $\to \text{Al}^{3+}$ .
2. Step 2
  Non-metals in Groups VI and VII GAIN electrons to reach a full shell. The charge equals $(8 - \text{group number})$ and is negative: O $\to \text{O}^{2-}$ (gains 2), Br $\to \text{Br}^{-}$ (gains 1).
Answer
$\text{K}^{+}$ , $\text{Ca}^{2+}$ , $\text{Al}^{3+}$ , $\text{O}^{2-}$ , $\text{Br}^{-}$ .
Examiner tip
Group I/II/III metals form +1/+2/+3 cations; Group V/VI/VII non-metals form −3/−2/−1 anions. The sign tells you whether electrons are lost (+) or gained (−).
3Formula of magnesium chloride by balancing charges
Building confidence• Adapted from 0620/22 May/Jun 2024 Q5• formula, charge balance
Question
Work out the formula of the compound formed between magnesium and chlorine.
Step-by-step solution
1. Step 1
  Identify the ions: Mg is in Group II → forms $\text{Mg}^{2+}$ . Cl is in Group VII → forms $\text{Cl}^{-}$ .
2. Step 2
  The compound must be electrically neutral, so total positive charge must equal total negative charge. One $\text{Mg}^{2+}$ carries $+2$ , but one $\text{Cl}^{-}$ carries only $-1$ , so TWO chloride ions are needed to balance.
3. Step 3
  Two $\text{Cl}^{-}$ give $-2$ , which cancels the $+2$ of one $\text{Mg}^{2+}$ . The formula is therefore $\text{MgCl}_{2}$ .
  $\text{Mg}^{2+} + 2\text{Cl}^{-} \to \text{MgCl}_{2}$
Answer
$\text{MgCl}_{2}$
Examiner tip
Use the 'swap and balance' idea but always check the charges cancel. $\text{MgCl}$ scores zero — the charges $+2$ and $-1$ do not balance.
4Dot-and-cross diagram for magnesium oxide
Building confidence• dot-and-cross, MgO
Question
Describe the dot-and-cross diagram and electron transfer for magnesium oxide, $\text{MgO}$ .
Step-by-step solution
1. Step 1
  Magnesium ( $2,8,2$ ) has two outer electrons (shown as crosses). Oxygen ( $2,6$ ) has six outer electrons (shown as dots) and needs two more.
2. Step 2
  Both of magnesium's outer electrons are transferred to the oxygen atom: Mg loses 2e to become $\text{Mg}^{2+}$ ( $2,8$ ); O gains 2e to become $\text{O}^{2-}$ ( $2,8$ ).
  $\text{Mg} \to \text{Mg}^{2+} + 2e^{-};\quad \text{O} + 2e^{-} \to \text{O}^{2-}$
3. Step 3
  Draw the cation with NO outer electrons and a square bracket with charge $2+$ outside; draw the anion with eight outer electrons (the two original lone pairs plus the two transferred electrons drawn as crosses) in a square bracket with charge $2-$ . Charges balance $1:1$ , so the formula is $\text{MgO}$ .
Answer
$\text{Mg}^{2+}$ (no outer electrons, $[ \,]^{2+}$ ) and $\text{O}^{2-}$ (full outer shell of 8 with the 2 transferred electrons shown as crosses, $[ \,]^{2-}$ ). Two electrons transferred; formula $\text{MgO}$ .
Examiner tip
For ionic dot-and-cross diagrams: show only the OUTER shell, use square brackets, write the charge outside, and the cation has an empty outer shell. Mark which electrons came from the metal (crosses) on the anion.
5Explaining properties from the giant ionic lattice
Stretch• properties, structure
Question
Sodium chloride has a high melting point, conducts electricity when molten but not when solid, and is soluble in water. Explain each property using its structure and bonding.
Step-by-step solution
1. Step 1
  High melting point: NaCl is a giant ionic lattice with strong electrostatic forces of attraction between the many oppositely charged ions. A large amount of energy is needed to overcome these forces, so the melting point is high.
2. Step 2
  Does not conduct when solid: the ions are held in fixed positions in the lattice, so there are no mobile charge carriers — the ions cannot move to carry charge.
3. Step 3
  Conducts when molten (or aqueous): melting (or dissolving) frees the ions from their fixed positions, so the ions are now mobile and can move to carry the electric current.
4. Step 4
  Soluble in water: the polar water molecules attract and separate the ions from the lattice, surrounding them, so the compound dissolves.
Answer
High melting point: strong electrostatic forces in a giant lattice need lots of energy to break. Solid does not conduct: ions are fixed/cannot move. Molten or aqueous conducts: ions are free to move and carry charge. Soluble: water molecules separate and surround the ions.
Examiner tip
Each property must be LINKED to the structure. The key conduction idea is mobile vs fixed ions, NOT mobile electrons (that is metallic bonding).
6Deducing a formula from polyatomic ions
Stretch• Paper 4 (Extended) structured style• formula, polyatomic
Question
Work out the formula of calcium nitrate, given that the calcium ion is $\text{Ca}^{2+}$ and the nitrate ion is $\text{NO}_{3}^{-}$ .
Step-by-step solution
1. Step 1
  Write the charges: $\text{Ca}^{2+}$ carries $+2$ ; $\text{NO}_{3}^{-}$ carries $-1$ .
2. Step 2
  Balance the charges so the compound is neutral: one $\text{Ca}^{2+}$ ( $+2$ ) needs two $\text{NO}_{3}^{-}$ ( $2 \times -1 = -2$ ).
3. Step 3
  Because there are two of a polyatomic ion, the whole nitrate ion is written in brackets with the subscript outside: $\text{Ca(NO}_{3})_{2}$ .
  $\text{Ca}^{2+} + 2\text{NO}_{3}^{-} \to \text{Ca(NO}_{3})_{2}$
Answer
$\text{Ca(NO}_{3})_{2}$
Examiner tip
When more than one polyatomic ion is needed, use brackets: $\text{Ca(NO}_{3})_{2}$ , never $\text{CaNO}_{32}$ or $\text{CaN}_{2}\text{O}_{6}$ .

Model Answers — Ions and Ionic Bonds

High-scoring sample answers for ions and ionic bonds on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 4 short-answer style1 mark
State what is meant by the term cation. (1 mark)
Model answer
A cation is a positively charged ion, formed when an atom loses one or more electrons.
Why this scores
One mark for 'positive ion' / 'positively charged ion'. Mentioning loss of electrons reinforces the point but is not essential for the mark.
Question 2
Paper 4 short-answer style2 marks
State the charge on the ion formed by sodium and the charge on the ion formed by chlorine, and explain how each ion is formed in terms of electrons. (2 marks)
Model answer
Sodium forms a $\text{Na}^{+}$ ion by losing one electron; chlorine forms a $\text{Cl}^{-}$ ion by gaining one electron.
Why this scores
Two marks: (1) sodium $+1$ by losing an electron; (2) chlorine $-1$ by gaining an electron. The electron lost by sodium is the one gained by chlorine.
Question 3
Paper 4 structured style3 marks
Define an ionic bond. (3 marks)
Model answer
An ionic bond is the strong electrostatic force of attraction between oppositely charged ions (a positive ion and a negative ion), formed by the transfer of electrons from a metal atom to a non-metal atom.
Why this scores
Three marks: (1) electrostatic attraction; (2) between oppositely charged ions / positive and negative ions; (3) formed by transfer of electrons (metal → non-metal). 'Electrostatic' is the key examiner word — 'attraction' alone is not enough.
Question 4
Paper 4 (Extended) structured style4 marks
Magnesium reacts with chlorine to form magnesium chloride. Deduce the formula of magnesium chloride and explain, in terms of ion charges, why this is the formula. (4 marks)
Model answer
The formula is $\text{MgCl}_{2}$ . Magnesium is in Group II, so it forms a $\text{Mg}^{2+}$ ion by losing two electrons. Chlorine is in Group VII, so it forms a $\text{Cl}^{-}$ ion by gaining one electron. Because the compound must be electrically neutral, the total positive and negative charges must balance: one $\text{Mg}^{2+}$ ( $+2$ ) needs two $\text{Cl}^{-}$ ions ( $2 \times -1 = -2$ ) to cancel its charge, giving $\text{MgCl}_{2}$ .
Why this scores
Four marks: (1) correct formula $\text{MgCl}_{2}$ ; (2) $\text{Mg}^{2+}$ ; (3) $\text{Cl}^{-}$ ; (4) charges must balance, so two chloride ions are needed. Writing 'MgCl' loses the formula mark.
Question 5
Paper 4 (Extended) structured style5 marks
Explain why solid sodium chloride does not conduct electricity, but it does conduct when it is molten or dissolved in water. (5 marks)
Model answer
Sodium chloride is made of ions ( $\text{Na}^{+}$ and $\text{Cl}^{-}$ ) and electrical conduction requires mobile (free-moving) charged particles to carry the current. In the solid, the ions are held in fixed positions in the giant ionic lattice by the strong electrostatic forces, so they cannot move and the solid does not conduct. When the compound is melted (molten) or dissolved in water (aqueous), the lattice breaks down and the ions become free to move. These mobile ions can then carry the charge through the liquid, so molten or aqueous sodium chloride conducts electricity.
Why this scores
Five marks across: conduction needs mobile charged particles/ions; solid has ions in fixed positions; so solid cannot conduct; melting/dissolving frees the ions; mobile ions carry the charge so it conducts. Do NOT say electrons move — it is the IONS that carry charge.
Question 6
Paper 4 (Extended) structured style6 marks
Magnesium oxide is an ionic compound. Describe its structure and use it to explain why magnesium oxide has a high melting point and why it is often soluble compounds of this type that conduct when dissolved in water. (6 marks)
Model answer
Magnesium oxide consists of $\text{Mg}^{2+}$ and $\text{O}^{2-}$ ions arranged in a giant ionic lattice — a regular, repeating three-dimensional arrangement of alternating oppositely charged ions. The ions are held together by strong electrostatic forces of attraction acting in all directions throughout the lattice. Because these forces are strong and there are very many of them, a large amount of (heat) energy is needed to overcome them and separate the ions, which is why magnesium oxide has a very high melting point (its charges are $2+$ and $2-$ , so the attraction is especially strong). In the solid the ions are locked in fixed positions and cannot move, so it does not conduct. However, when an ionic compound dissolves in water, the water molecules separate the ions and surround them, so the ions become free to move; these mobile ions can carry an electric charge, which is why solutions of ionic compounds conduct electricity.
Why this scores
Up to 6 marks: giant ionic lattice; regular/alternating oppositely charged ions; strong electrostatic forces (in all directions); large energy needed to overcome them → high melting point; ions fixed in solid (no conduction); dissolving frees the ions so mobile ions conduct. The high charges ( $2+/2-$ ) on MgO explain its especially high melting point.

Key Formulae — Ions and Ionic Bonds

The formulae you need to memorise for ions and ionic bonds on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Common ion charges from the Periodic Table
$\text{Group I: } +1;\ \text{II: } +2;\ \text{III: } +3;\ \text{V: } -3;\ \text{VI: } -2;\ \text{VII: } -1$
When to use
Predicting ion charges of main-group elements.

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 0620 sitting.

Ion
Examiner keyword
An atom (or group of atoms) that carries an electric charge because it has lost or gained one or more electrons.
Cation / Anion
Examiner keyword
Cation: a positive ion formed when an atom LOSES electrons (typically a metal). Anion: a negative ion formed when an atom GAINS electrons (typically a non-metal).
Ionic bond
Examiner keyword
The strong electrostatic force of attraction between oppositely charged ions, formed by the transfer of electrons from a metal atom to a non-metal atom.
Giant ionic lattice
Examiner keyword
A regular, repeating three-dimensional arrangement of alternating positive and negative ions held together by strong ionic bonds in all directions.
Electrostatic attraction
Examiner keyword
The force of attraction between particles of opposite electric charge — the force that holds positive and negative ions together in an ionic compound.

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 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying ionic bonds form by sharing electrons.
0620/42 — recurring
Why it happens
Confusing ionic bonding with covalent bonding.
How to avoid it
Ionic = TRANSFER of electrons (metal → non-metal), forming charged ions. Covalent = SHARING of electrons between non-metals.
✕Writing $\text{MgCl}$ for magnesium chloride.
Why it happens
Forgetting that the charges must balance.
How to avoid it
Total positive charge must equal total negative charge. $\text{Mg}^{2+}$ ( $+2$ ) needs TWO $\text{Cl}^{-}$ ( $-1$ each) → $\text{MgCl}_{2}$ .
✕Saying solid ionic compounds conduct electricity.
Why it happens
Knowing ionic compounds 'conduct' without linking it to mobile ions.
How to avoid it
Conduction needs MOBILE charge carriers. In a solid the ions are FIXED in the lattice; only molten or aqueous ionic compounds (free-moving ions) conduct.
✕Saying ionic compounds conduct because electrons are free to move.
Why it happens
Mixing up ionic conduction with metallic conduction.
How to avoid it
In ionic compounds it is the IONS that move and carry charge, not electrons. Delocalised electrons carry charge in metals, not ionic substances.

Ions and Ionic Bonds — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Group

Charge formed

+1

+2

+3

-3

-2

-1

already full — usually no ions

Ionic bond. The strong electrostatic ATTRACTION between oppositely charged ions.

Forming sodium chloride.

Na atom loses 1 e⁻ → Na⁺.
Cl atom gains that e⁻ → Cl⁻.
Na⁺ and Cl⁻ attract each other electrostatically.
Many Na⁺ and Cl⁻ ions arrange themselves in a 3D pattern — a giant ionic lattice — alternating + and − ions.

Sodium gives its single outer electron to chlorine; both reach a full outer shell and the resulting ions attract.

Drawing dot-and-cross diagrams. Show outer-shell electrons only.

For NaCl, you'd draw Na surrendering its outer × to Cl, then Cl shown with 8 dots (full shell). The two ions are drawn next to each other in square brackets, with their charges.

Stoichiometry from charges. Total positive charge = total negative charge in the formula.

Na⁺ and Cl⁻ → NaCl (one of each).
Mg²⁺ and Cl⁻ → MgCl₂ (one Mg²⁺ balances two Cl⁻).
Al³⁺ and O²⁻ → Al₂O₃ (lowest common multiple is 6: 2×3 = 3×2).

Worked. Calcium reacting with bromine.

Ca: $2, 8, 8, 2$ → Ca²⁺.
Br: gains 1 e⁻ → Br⁻.
Need 2 Br⁻ to balance one Ca²⁺ → CaBr₂.

Two 1− bromide ions are needed to cancel one 2+ calcium ion, so the formula is CaBr₂.

Property

Ionic compound

Reason

Melting / boiling point

High (NaCl 801 °C)

Strong electrostatic forces throughout the lattice

Structure

Crystalline

Regular repeating lattice gives a regular shape

Solubility in water

Often soluble

Polar water molecules pull ions out of the lattice

Conducts as a solid

Ions are locked in the lattice and cannot move

Conducts when molten

Yes

Lattice is broken, so ions are free to move

Conducts when dissolved

Yes

Ions are separated by water and free to move

Ions and Ionic Bonds

Short Notes - Ions and Ionic Bonds

Detailed Study Notes

At a glance

What you’ll learn

How ions form

Ionic bonding and the lattice

Properties of ionic compounds

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Formation of NaCl\text{NaCl}NaCl from atoms

2Predicting ion charges from group number

3Formula of magnesium chloride by balancing charges

4Dot-and-cross diagram for magnesium oxide

5Explaining properties from the giant ionic lattice

6Deducing a formula from polyatomic ions

Common ion charges from the Periodic Table

Ion

Cation / Anion

Ionic bond

Giant ionic lattice

Electrostatic attraction

✕Saying ionic bonds form by sharing electrons.

✕Writing MgCl\text{MgCl}MgCl for magnesium chloride.

✕Saying solid ionic compounds conduct electricity.

✕Saying ionic compounds conduct because electrons are free to move.

Ions and Ionic Bonds — frequently asked questions

Ions and Ionic Bonds

Short Notes - Ions and Ionic Bonds

Detailed Study Notes

At a glance

What you’ll learn

How ions form

Ionic bonding and the lattice

Properties of ionic compounds

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Formation of NaCl\text{NaCl}NaCl from atoms

2Predicting ion charges from group number

3Formula of magnesium chloride by balancing charges

4Dot-and-cross diagram for magnesium oxide

5Explaining properties from the giant ionic lattice

6Deducing a formula from polyatomic ions

Common ion charges from the Periodic Table

Ion

Cation / Anion

Ionic bond

Giant ionic lattice

Electrostatic attraction

✕Saying ionic bonds form by sharing electrons.

✕Writing MgCl\text{MgCl}MgCl for magnesium chloride.

✕Saying solid ionic compounds conduct electricity.

✕Saying ionic compounds conduct because electrons are free to move.

Ions and Ionic Bonds — frequently asked questions

1Formation of $\text{NaCl}$ from atoms

✕Writing $\text{MgCl}$ for magnesium chloride.

1Formation of $\text{NaCl}$ from atoms

✕Writing $\text{MgCl}$ for magnesium chloride.