Ionic Bonding

Specification Reference Physical Chemistry: Chemical bonding 3.2

Quick Notes

Ionic bonding is the electrostatic attraction between oppositely charged ions – positively charged cations and negatively charged anions.
Usually, metals lose electrons to become cations, and non-metals gain electrons to become anions in reactions.
Ionic compounds form giant lattice structures held together by strong forces of electrostatic attraction.

Full Notes

Ionic bonding has been outlined with more background theory and detail at this page.
This page is just what you need to know for CIE A-level Chemistry :)

What is Ionic Bonding?

Ionic bonding is the strong electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions) in an ionic compound.

In solid ionic compounds, ions are arranged in a regular 3D structure called an ionic lattice.

The closer oppositely charged ions are to each other in the lattice, the stronger the strength of attraction.
The greater the charge of the ions, the stronger the strength of attraction.

Example Sodium chloride (NaCl) is made up of Na⁺ and Cl⁻ ions attracted to each other in a giant repeating structure.

CIE A-Level Chemistry diagram showing sodium chloride lattice with Na+ and Cl- ions arranged in a giant repeating structure.

Matt’s exam tip

Sometimes lattice structures can be represented using unit cells with ions shown as corners of a cube.

CIE A-Level Chemistry unit cell representation of NaCl lattice structure with ions shown at cube corners.

However it is important to understand in reality, ions are actually touching each other.

Formation of Ions

The most stable configuration for most atoms is to have eight electrons in their outermost shell (octet rule). Note, transition metals are an exception to this (see Transition Metals).

Atoms of elements can often lose or gain electrons to achieve a full outer shell of electrons.

Metals (Groups 1, 2, 3) usually lose electrons to form positive ions (cations).
- Group 1 metals form ions with 1+ charge (e.g. Na⁺)
- Group 2 metals form ions with 2+ charge (e.g. Mg²⁺)
- Group 3 metals form ions with 3+ charge (e.g. Al³⁺)
Non-metals (Groups 5, 6, 7) usually gain electrons to form negative ions (anions).
- Group 5 non-metals form ions with 3− charge (e.g. N³⁻)
- Group 6 non-metals form ions with 2− charge (e.g. O²⁻)
- Group 7 non-metals form ions with 1− charge (e.g. Cl⁻)

Examples of Ionic Compounds

Sodium chloride (NaCl)

CIE A-Level Chemistry diagram showing sodium losing one electron and chlorine gaining one electron to form NaCl lattice.

Sodium atom (Na) loses one electron → Na⁺
Chlorine atom (Cl) gains one electron → Cl⁻
Ions combine to form NaCl, held in a lattice.

Magnesium oxide (MgO)

CIE A-Level Chemistry diagram showing magnesium losing two electrons and oxygen gaining two electrons to form MgO.

Mg loses two electrons → Mg²⁺
O gains two electrons → O²⁻
1:1 ratio gives the formula MgO.

Calcium fluoride (CaF₂)

CIE A-Level Chemistry diagram showing calcium losing two electrons and two fluorine atoms gaining one electron each to form CaF2.

Ca loses two electrons → Ca²⁺
Each F gains one electron → F⁻
Two F⁻ ions needed to balance one Ca²⁺ → formula is CaF₂.

Physical Properties of Ionic Compounds

High melting and boiling points due to strong electrostatic forces.
Conduct electricity when molten or in aqueous solution (ions are free to move and can carry charge).
Do not conduct electricity when solid (ions fixed in place).

Summary

Ionic bonding is the electrostatic attraction between cations and anions.
Metals lose electrons to form cations; non-metals gain electrons to form anions.
Ionic compounds form giant lattices with strong electrostatic attractions.
Ion charge and ion distance affect bond strength.
Properties: high melting/boiling points, conduct electricity when molten/in solution, non-conductive when solid.