# Ionic Bonding

> Edexcel International GCSE Chemistry · 4CH1 2017
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-ionic-bonding/

This guide covers all core ionic bonding content for the Edexcel IGCSE 4CH1 specification, including ion formation, formula writing, dot-and-cross diagrams, and ionic compound properties, with exam-aligned worked examples.

**Prerequisites:** [Atomic structure and electron configuration](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-atomic-structure/); [Periodic table group trends](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-periodic-table/)

## Learning objectives

- Explain how ions form via electron loss (metals) or gain (non-metals)
- Recall correct charges for common main group, transition metal, and polyatomic ions
- Write balanced, charge-neutral formulae for ionic compounds
- Draw valid dot-and-cross diagrams for ionic compounds formed from main group elements
- Define ionic bonding as electrostatic attraction between oppositely charged ions
- Explain the melting/boiling point and conductivity properties of giant ionic lattices

## Ion Formation & Common Ion Charges

Ions form when atoms lose or gain electrons to achieve a full outer electron shell (stable noble gas configuration). Metals lose electrons to form positively charged cations, while non-metals gain electrons to form negatively charged anions.

**Ion** — A charged particle formed by the loss or gain of one or more electrons from an atom

For main group elements, ion charge can be predicted from their group number: Group 1 → +1, Group 2 → +2, Group 3 → +3, Group 5 → -3, Group 6 → -2, Group 7 → -1. Transition metal charges ($Ag^+$, $Cu^{2+}$, $Fe^{2+}$, $Fe^{3+}$, $Pb^{2+}$, $Zn^{2+}$) and polyatomic ion charges ($H^+$, $OH^-$, $NH_4^+$, $CO_3^{2-}$, $NO_3^-$, $SO_4^{2-}$) must be memorized.

**Worked example:** Predict the charge of ions formed by (a) magnesium (Group 2), (b) sulfur (Group 6), (c) silver.

1. Group 2 metals lose 2 outer electrons to form a full outer shell, so the magnesium ion has a +2 charge: $Mg^{2+}$
2. Group 6 non-metals gain 2 electrons to form a full outer shell, so the sulfide ion has a -2 charge: $S^{2-}$
3. Silver ion charge is a required recall value, so the silver ion has a +1 charge: $Ag^+$

**Check your understanding**

1. What is the charge of an aluminium ion (Group 3)?

   *Why:* Group 3 metals lose 3 outer electrons to form 3+ ions.

2. What is the charge of a carbonate ion?

   *Why:* Carbonate is a polyatomic ion with a fixed 2- charge that must be memorized.

## Writing Charge-Neutral Ionic Formulae

Ionic compounds are electrically neutral overall, so the total positive charge from cations must equal the total negative charge from anions. Use multipliers to balance charges, and wrap polyatomic ions in brackets if you need more than one copy of the ion in the formula.

**Polyatomic ion** — A charged particle made of two or more covalently bonded atoms

**Worked example:** Write the formula for (a) aluminium oxide, (b) ammonium sulfate.

1. For aluminium oxide: Identify ion charges: $Al^{3+}$ (Group 3) and $O^{2-}$ (Group 6). The lowest common multiple of 3 and 2 is 6, so you need 2 $Al^{3+}$ ions (total +6 charge) and 3 $O^{2-}$ ions (total -6 charge). The formula is $Al_2O_3$.
2. For ammonium sulfate: Identify ion charges: $NH_4^+$ (polyatomic) and $SO_4^{2-}$ (polyatomic). You need 2 $NH_4^+$ ions to balance the 2- charge of 1 sulfate ion. Wrap ammonium in brackets because it is a polyatomic ion with a multiplier. The formula is $(NH_4)_2SO_4$.

> **tip**
>
> Always double-check that total positive charge equals total negative charge in your formula to avoid common balancing errors.

## Dot-and-Cross Diagrams for Ionic Compounds

Dot-and-cross diagrams show electron transfer between metals and non-metals to form ionic compounds. Only outer electron shells are drawn, with electrons from one element shown as dots and electrons from the other as crosses. Each resulting ion is enclosed in square brackets, with its overall charge written at the top right outside the brackets.

**Ionic dot-and-cross diagram** — A diagram showing the transfer of outer electrons between atoms to form oppositely charged ions, with electrons from different atoms distinguished by dots and crosses

**Worked example:** Draw a dot-and-cross diagram for the formation of magnesium chloride ($MgCl_2$).

1. Draw the outer electron shells of 1 magnesium atom (2 outer electrons, drawn as dots) and 2 chlorine atoms (7 outer electrons each, drawn as crosses).
2. Show the magnesium atom losing both outer electrons, one transferred to each chlorine atom.
3. Draw the resulting ions: $Mg^{2+}$ with an empty outer shell in square brackets, charge +2 outside. Each $Cl^-$ ion has a full outer shell of 8 electrons (7 crosses + 1 dot) in square brackets, charge -1 outside.

> **warning**
>
> Never forget square brackets and charge labels around ions in dot-and-cross diagrams, as these are required to score full marks in exams.

## Ionic Bonding & Giant Ionic Lattice Properties

Ionic bonding is the strong electrostatic attraction between oppositely charged ions, which holds ions together in a regular 3D structure called a giant ionic lattice.

**Giant ionic lattice** — A regular three-dimensional arrangement of oppositely charged ions held together by strong ionic bonds

Giant ionic lattices have high melting and boiling points because the strong electrostatic forces between oppositely charged ions require a large amount of heat energy to overcome. Solid ionic compounds do not conduct electricity, as ions are fixed in position in the lattice and cannot move to carry charge. When molten (melted) or dissolved in water (aqueous solution), ions are free to move, so the compound conducts electricity.

**Worked example:** Explain why sodium chloride has a melting point of 801°C.

1. Sodium chloride is a giant ionic lattice made of $Na^+$ and $Cl^-$ ions.
2. Strong electrostatic forces of attraction exist between the oppositely charged ions.
3. A large amount of heat energy is required to overcome these strong forces, resulting in a high melting point.

**Exam command terms**

- **Explain high melting point of ionic compound** — You must link the property to strong electrostatic forces between oppositely charged ions that require large amounts of energy to overcome. *(Magnesium oxide has strong electrostatic forces between $Mg^{2+}$ and $O^{2-}$ ions that require lots of energy to break, so it has a high melting point.)*

- **Write ionic formula** — Ensure total charge is zero, use brackets around polyatomic ions if you need more than one copy. *(Calcium hydroxide has formula $Ca(OH)_2$, not $CaOH_2$.)*

## Common pitfalls

- **Wrong:** Forgetting brackets around polyatomic ions with multipliers (e.g. writing $NH_42SO_4$ instead of $(NH_4)_2SO_4$)
  - Why it fails: Omitting brackets incorrectly suggests only the last atom in the polyatomic ion is multiplied, leading to an invalid formula.
  - Correct: Always wrap polyatomic ions in brackets if you need more than one copy of the ion to balance charge.
- **Wrong:** Drawing dot-and-cross diagrams without square brackets and charge labels around ions
  - Why it fails: This fails to show that electrons are transferred (not shared) and that the resulting particles are charged.
  - Correct: Enclose every ion in square brackets, with the overall charge written at the top right outside the brackets.
- **Wrong:** Stating solid ionic compounds conduct electricity
  - Why it fails: Ions are fixed in position in the solid giant lattice and cannot move to carry charge.
  - Correct: Only state ionic compounds conduct electricity when molten or dissolved in water, as ions are free to move in these states.
- **Wrong:** Changing ion charges to balance formulae instead of adjusting ion counts
  - Why it fails: Ion charges are fixed for a given ion and cannot be altered.
  - Correct: Adjust the number of each ion until the total positive charge equals the total negative charge.
- **Wrong:** Using an incorrect charge for transition metal or polyatomic ions
  - Why it fails: These charges are not derivable from the periodic table and must be recalled.
  - Correct: Memorize all required ion charges listed in the specification to avoid this common error.

## Cheatsheet

| Ion Category | Ion Examples | Charge | Example Formula |
| --- | --- | --- | --- |
| Group 1 metals | $Li^+, Na^+, K^+$ | +1 | $NaCl$ |
| Group 2 metals | $Mg^{2+}, Ca^{2+}$ | +2 | $MgO$ |
| Group 3 metals | $Al^{3+}$ | +3 | $Al_2O_3$ |
| Group 7 non-metals | $F^-, Cl^-, Br^-$ | -1 | $KBr$ |
| Group 6 non-metals | $O^{2-}, S^{2-}$ | -2 | $Na_2S$ |
| Group 5 non-metals | $N^{3-}$ | -3 | $Mg_3N_2$ |
| Required transition metals | $Ag^+, Cu^{2+}, Fe^{2+}, Fe^{3+}, Pb^{2+}, Zn^{2+}$ | Varies (recall) | $CuSO_4$ |
| Required polyatomic ions | $H^+, OH^-, NH_4^+, CO_3^{2-}, NO_3^-, SO_4^{2-}$ | Varies (recall) | $(NH_4)_2SO_4$ |

## What's next

Now that you have mastered core ionic bonding concepts, you are ready to progress to related topics in the Principles of Chemistry unit. Next, you will study covalent bonding, which forms when non-metal atoms share electrons rather than transfer them, and learn to compare the properties of ionic and simple covalent compounds. Following that, you will cover metallic bonding, the third main type of chemical bonding, before moving to electrolysis, where you will apply your knowledge of ionic compound conductivity to explain electrolytic reactions. These topics are frequently tested together in structured exam questions, so ensure you are confident with ionic formula writing and dot-and-cross diagrams before proceeding, as they form the foundation for all bonding content in the course.

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