Ionic Bonding

This lesson covers ionic bonding — the transfer of electrons between metals and non-metals to form charged particles called ions — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how ionic bonds form, draw dot-and-cross diagrams, and explain why ions have charges.

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What Is Ionic Bonding?

Ionic bonding is the electrostatic attraction between oppositely charged ions. It occurs between a metal and a non-metal.

The process involves the transfer of electrons:

1. A metal atom loses one or more electrons from its outer shell.
2. A non-metal atom gains those electrons into its outer shell.
3. Both atoms now have a full outer shell — a stable electron configuration like a noble gas.
4. The metal becomes a positive ion (cation) and the non-metal becomes a negative ion (anion).
5. The opposite charges attract each other strongly — this is the ionic bond.

Exam Tip: Always state that ionic bonding involves the transfer of electrons, not the sharing of electrons. Sharing is covalent bonding.

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Why Do Atoms Form Ions?

Atoms are most stable when they have a full outer electron shell. This is the same electron configuration as a noble gas (Group 0). Atoms achieve this by:

• Metals — losing electrons to empty their outer shell, revealing the full shell beneath.
• Non-metals — gaining electrons to fill their outer shell.

Atom	Group	Electrons Lost/Gained	Ion Formed	Noble Gas Configuration
Sodium (Na)	1	Loses 1 electron	Na⁺	Neon (2, 8)
Magnesium (Mg)	2	Loses 2 electrons	Mg²⁺	Neon (2, 8)
Aluminium (Al)	3	Loses 3 electrons	Al³⁺	Neon (2, 8)
Chlorine (Cl)	7	Gains 1 electron	Cl⁻	Argon (2, 8, 8)
Oxygen (O)	6	Gains 2 electrons	O²⁻	Neon (2, 8)

Exam Tip: The charge on an ion can be predicted from the group number. Group 1 metals form +1 ions, Group 2 form +2 ions, Group 6 non-metals form −2 ions, and Group 7 non-metals form −1 ions.

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Dot-and-Cross Diagrams for Ionic Bonding

A dot-and-cross diagram shows how electrons are transferred during ionic bonding. Dots represent electrons from one atom and crosses represent electrons from the other.

Example 1: Sodium Chloride (NaCl)

graph LR
    A["Na atom<br/>2, 8, 1"] -->|"Loses 1 electron"| B["Na⁺ ion<br/>2, 8"]
    C["Cl atom<br/>2, 8, 7"] -->|"Gains 1 electron"| D["Cl⁻ ion<br/>2, 8, 8"]
    A -->|"1 electron transferred"| C

    style A fill:#2980b9,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff

• Sodium (2, 8, 1) loses its 1 outer electron → Na⁺ (2, 8).
• Chlorine (2, 8, 7) gains that electron → Cl⁻ (2, 8, 8).
• Both ions now have a full outer shell.
• The formula is NaCl because one sodium ion bonds with one chloride ion.

Example 2: Magnesium Oxide (MgO)

• Magnesium (2, 8, 2) loses 2 outer electrons → Mg²⁺ (2, 8).
• Oxygen (2, 6) gains those 2 electrons → O²⁻ (2, 8).
• The formula is MgO — the charges balance: 2+ and 2−.

Example 3: Magnesium Chloride (MgCl₂)

• Magnesium needs to lose 2 electrons but chlorine can only gain 1.
• Therefore, magnesium transfers one electron to each of two chlorine atoms.
• Mg²⁺ + 2Cl⁻ → MgCl₂.

Example 4: Sodium Oxide (Na₂O)

• Oxygen needs to gain 2 electrons but sodium can only lose 1.
• Two sodium atoms each transfer one electron to one oxygen atom.
• 2Na⁺ + O²⁻ → Na₂O.

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Drawing Dot-and-Cross Diagrams — Rules

When drawing dot-and-cross diagrams for ionic compounds in the exam:

1. Show only the outer shell electrons (unless the question asks for all shells).
2. Use dots for electrons from one atom and crosses for the other.
3. Draw square brackets around each ion.
4. Write the charge of each ion outside the brackets (e.g. [Na]⁺ or [Cl]⁻).
5. Show the transferred electron changing from a dot to a cross (or vice versa) so the examiner can see it came from the other atom.

Exam Tip: Always include square brackets and charges on your dot-and-cross diagrams. Missing brackets or charges will lose marks even if the electron arrangement is correct.

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Working Out Formulae of Ionic Compounds

To work out the formula, make sure the total positive charge equals the total negative charge so the compound is neutral overall.

Positive Ion	Negative Ion	Balancing	Formula
Na⁺	Cl⁻	1 × (+1) + 1 × (−1) = 0	NaCl
Mg²⁺	Cl⁻	1 × (+2) + 2 × (−1) = 0	MgCl₂
Na⁺	O²⁻	2 × (+1) + 1 × (−2) = 0	Na₂O
Ca²⁺	F⁻	1 × (+2) + 2 × (−1) = 0	CaF₂
Al³⁺	O²⁻	2 × (+3) + 3 × (−2) = 0	Al₂O₃

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Summary

• Ionic bonding is the strong electrostatic attraction between oppositely charged ions, formed by the transfer of electrons from a metal to a non-metal.
• Metals lose electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions).
• Both ions achieve a full outer shell (noble gas configuration).
• Dot-and-cross diagrams show the electron transfer — always include square brackets and charges.
• The formula of an ionic compound is found by balancing the charges so the compound is electrically neutral.