Ionic Bonding

This lesson covers the formation of ionic bonds as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry — Structure, Bonding and Properties. You need to understand how ionic bonds form through the transfer of electrons and how to represent ionic compounds using dot-and-cross diagrams.

What Is Ionic Bonding?

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

Metals are on the left of the periodic table — they have relatively few electrons in their outer shell and tend to lose electrons.
Non-metals are on the right of the periodic table — they need only a few electrons to complete their outer shell and tend to gain electrons.

When a metal atom reacts with a non-metal atom:

The metal atom loses one or more electrons from its outer shell, forming a positive ion (cation).
The non-metal atom gains one or more electrons into its outer shell, forming a negative ion (anion).
Both ions now have a full outer shell — the same electronic structure as a noble gas.

Exam Tip: Always state that the bond is due to the "strong electrostatic attraction between oppositely charged ions." This is the definition the examiner is looking for.

Why Do Atoms Form Ions?

Atoms are most stable when they have a full outer shell of electrons (like the noble gases). Atoms with incomplete outer shells will lose, gain or share electrons to achieve this stable arrangement.

Group 1 metals (e.g. Na, K) lose 1 electron → form ions with a +1 charge (Na⁺, K⁺).
Group 2 metals (e.g. Mg, Ca) lose 2 electrons → form ions with a +2 charge (Mg²⁺, Ca²⁺).
Group 6 non-metals (e.g. O, S) gain 2 electrons → form ions with a -2 charge (O²⁻, S²⁻).
Group 7 non-metals (e.g. Cl, Br) gain 1 electron → form ions with a -1 charge (Cl⁻, Br⁻).

Exam Tip: The charge on the ion is determined by the group number. Group 1 → +1, Group 2 → +2, Group 6 → -2, Group 7 → -1.

Dot-and-Cross Diagrams for Ionic Compounds

Dot-and-cross diagrams show what happens to the outer-shell electrons when ionic bonds form. Electrons from the metal atom are shown as dots (•) and electrons from the non-metal atom are shown as crosses (×) — or vice versa. This allows you to track which atom each electron originally belonged to.

Example 1: Sodium Chloride (NaCl)

Sodium (Na): Electronic structure 2, 8, 1 — has 1 electron in its outer shell.
Chlorine (Cl): Electronic structure 2, 8, 7 — has 7 electrons in its outer shell.

What happens:

Sodium loses 1 electron → Na⁺ (electronic structure 2, 8).
Chlorine gains 1 electron → Cl⁻ (electronic structure 2, 8, 8).

The dot-and-cross diagram shows:

Na⁺ with an empty outer shell (all electrons transferred), drawn in square brackets with a + charge.
Cl⁻ with 8 electrons in its outer shell (7 crosses + 1 dot from sodium), drawn in square brackets with a - charge.

The formula is NaCl — one sodium ion to one chloride ion.

Example 2: Magnesium Oxide (MgO)

Magnesium (Mg): Electronic structure 2, 8, 2 — has 2 electrons in its outer shell.
Oxygen (O): Electronic structure 2, 6 — has 6 electrons in its outer shell.

What happens:

Magnesium loses 2 electrons → Mg²⁺ (electronic structure 2, 8).
Oxygen gains 2 electrons → O²⁻ (electronic structure 2, 8).

The dot-and-cross diagram shows:

Mg²⁺ drawn in square brackets with a 2+ charge.
O²⁻ with 8 electrons in its outer shell (6 crosses + 2 dots from magnesium), drawn in square brackets with a 2- charge.

The formula is MgO — one magnesium ion to one oxide ion.

Example 3: Calcium Chloride (CaCl₂)

Calcium (Ca): Electronic structure 2, 8, 8, 2 — has 2 electrons in its outer shell.
Chlorine (Cl): Electronic structure 2, 8, 7 — each chlorine has 7 electrons in its outer shell.

What happens:

Calcium loses 2 electrons → Ca²⁺ (electronic structure 2, 8, 8).
Each chlorine atom gains 1 electron → 2 × Cl⁻ (electronic structure 2, 8, 8 each).

Because calcium loses 2 electrons and each chlorine only needs 1, two chlorine atoms are needed for every one calcium atom.

The formula is CaCl₂.

Exam Tip: When drawing dot-and-cross diagrams, always put square brackets around each ion and write the charge outside the bracket. Don't forget the charge!

The Giant Ionic Lattice

Ionic compounds do not exist as individual pairs of ions. Instead, the ions are arranged in a regular, repeating three-dimensional pattern called a giant ionic lattice.

Each positive ion is surrounded by negative ions, and each negative ion is surrounded by positive ions.
The strong electrostatic forces act in all directions between the oppositely charged ions.
This arrangement maximises the attractive forces between oppositely charged ions and minimises the repulsive forces between ions of the same charge.

For example, in sodium chloride:

Each Na⁺ ion is surrounded by 6 Cl⁻ ions.
Each Cl⁻ ion is surrounded by 6 Na⁺ ions.
The structure extends in all three dimensions.

Formulae of Ionic Compounds

The formula of an ionic compound is determined by the need for the total positive charge to balance the total negative charge, so the overall compound is electrically neutral.

Metal Ion	Non-metal Ion	Formula	Reasoning
Na⁺	Cl⁻	NaCl	+1 and -1 balance
Mg²⁺	O²⁻	MgO	+2 and -2 balance
Ca²⁺	Cl⁻	CaCl₂	+2 needs 2 × (-1) to balance
Na⁺	O²⁻	Na₂O	2 × (+1) needed to balance -2
Mg²⁺	Cl⁻	MgCl₂	+2 needs 2 × (-1) to balance

Worked Example: Lithium Fluoride (LiF)

Question: Describe the formation of lithium fluoride (LiF) from its elements using a dot-and-cross diagram.

Answer:

Lithium (Li) has the electronic structure 2, 1 — it has 1 electron in its outer shell.
Fluorine (F) has the electronic structure 2, 7 — it has 7 electrons in its outer shell.
Lithium loses 1 electron → Li⁺ (electronic structure 2).
Fluorine gains 1 electron → F⁻ (electronic structure 2, 8).
The dot-and-cross diagram shows Li⁺ in square brackets with a + charge, and F⁻ with 8 electrons (7 crosses + 1 dot) in square brackets with a - charge.
The ions are held together by strong electrostatic attraction in a giant ionic lattice.
The formula is LiF.

Key Points

Ionic bonding occurs between metals and non-metals.
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).
An ionic bond is the strong electrostatic attraction between oppositely charged ions.
Ions are arranged in a giant ionic lattice — a regular, repeating 3D structure.
The formula of an ionic compound reflects the ratio of ions needed for overall electrical neutrality.

Exam Tip: A very common 6-mark question asks you to "Describe, with the aid of a diagram, the bonding in sodium chloride." You should: (1) state electrons are transferred, (2) draw a dot-and-cross diagram with correct charges, (3) state ions are held by strong electrostatic attraction, and (4) mention the giant ionic lattice.

Practice Questions

Describe what happens to the electrons when sodium reacts with chlorine to form sodium chloride.
Draw a dot-and-cross diagram for magnesium oxide (MgO).
Explain why the formula of calcium chloride is CaCl₂ and not CaCl.
What is meant by the term "ionic bond"?
Describe the structure of an ionic compound.
Draw a dot-and-cross diagram for lithium fluoride (LiF) and state the charges on each ion.