Ionic Bonding

This lesson covers ionic bonding, one of the three main types of chemical bonding you need to understand for the AQA GCSE Chemistry specification (4.2.1). Ionic bonding occurs between metals and non-metals and involves the transfer of electrons. Understanding how and why ions form is fundamental to explaining the properties of ionic compounds, which are covered in the next lesson.

What Is Ionic Bonding?

Ionic bonding is the electrostatic force of attraction between oppositely charged ions. It occurs when atoms transfer electrons from one to another, forming positive and negative ions that are held together by strong electrostatic forces.

Ionic bonds typically form between metals (which lose electrons) and non-metals (which gain electrons). The driving force behind ionic bonding is that atoms want to achieve a full outer shell of electrons — the same electron configuration as a noble gas. This is sometimes called the octet rule (having 8 electrons in the outer shell, or 2 for the first shell).

Exam Tip: The definition of ionic bonding is a common 1-mark question. Learn it precisely: "Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions." Do not say "between a metal and a non-metal" — that describes when it happens, not what it is.

How Ions Form

Metal Atoms Form Positive Ions (Cations)

Metal atoms have a small number of electrons in their outer shell (typically 1, 2, or 3). They achieve a full outer shell by losing these outer electrons. When a metal atom loses electrons, it has more protons than electrons, giving it an overall positive charge.

Metal	Group	Electrons Lost	Ion Formed	Electron Configuration
Sodium (Na)	1	1	Na+	2, 8 (same as neon)
Magnesium (Mg)	2	2	Mg2+	2, 8 (same as neon)
Aluminium (Al)	3	3	Al3+	2, 8 (same as neon)
Potassium (K)	1	1	K+	2, 8, 8 (same as argon)
Calcium (Ca)	2	2	Ca2+	2, 8, 8 (same as argon)

Non-Metal Atoms Form Negative Ions (Anions)

Non-metal atoms have outer shells that are close to being full (typically 5, 6, or 7 electrons). They achieve a full outer shell by gaining electrons. When a non-metal atom gains electrons, it has more electrons than protons, giving it an overall negative charge.

Non-Metal	Group	Electrons Gained	Ion Formed	Electron Configuration
Chlorine (Cl)	7	1	Cl-	2, 8, 8 (same as argon)
Oxygen (O)	6	2	O2-	2, 8 (same as neon)
Fluorine (F)	7	1	F-	2, 8 (same as neon)
Sulfur (S)	6	2	S2-	2, 8, 8 (same as argon)
Nitrogen (N)	5	3	N3-	2, 8 (same as neon)

Exam Tip: The charge on an ion is directly related to the group number. Group 1 metals form 1+ ions, Group 2 form 2+ ions, Group 6 non-metals form 2- ions, and Group 7 form 1- ions. This pattern is a quick way to predict the charge of any main group ion.

The Process of Ionic Bonding: Sodium Chloride

Let us consider the formation of sodium chloride (NaCl) as a worked example.

A sodium atom (Na) has the electron configuration 2, 8, 1. It has 1 electron in its outer shell.
A chlorine atom (Cl) has the electron configuration 2, 8, 7. It has 7 electrons in its outer shell and needs 1 more to fill it.
The sodium atom transfers its one outer electron to the chlorine atom.
Sodium becomes Na+ (electron configuration 2, 8) — a positive ion.
Chlorine becomes Cl- (electron configuration 2, 8, 8) — a negative ion.
The oppositely charged ions are attracted to each other by a strong electrostatic force of attraction — this is the ionic bond.

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"]
    B -->|"Electrostatic attraction"| E["NaCl<br/>Ionic compound"]
    D -->|"Electrostatic attraction"| E

    style A fill:#3498db,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#8e44ad,color:#fff
    style E fill:#f39c12,color:#fff

The Process of Ionic Bonding: Magnesium Oxide

Now consider magnesium oxide (MgO), where two electrons are transferred.

A magnesium atom (Mg) has the electron configuration 2, 8, 2. It has 2 electrons in its outer shell.
An oxygen atom (O) has the electron configuration 2, 6. It has 6 electrons in its outer shell and needs 2 more to fill it.
The magnesium atom transfers both outer electrons to the oxygen atom.
Magnesium becomes Mg2+ (electron configuration 2, 8).
Oxygen becomes O2- (electron configuration 2, 8).
The 2+ and 2- ions are strongly attracted to each other.

Dot-and-Cross Diagrams

In the exam you will be asked to draw dot-and-cross diagrams to show ionic bonding. These diagrams show only the outer shell electrons, using dots for one atom and crosses for the other.

Rules for Drawing Dot-and-Cross Diagrams

Draw the outer shell electrons of the metal atom using dots (or crosses).
Draw the outer shell electrons of the non-metal atom using the other symbol.
Show the electron being transferred by drawing it in the outer shell of the non-metal ion.
Put square brackets around each ion.
Write the charge of each ion outside the brackets (top right).
Label which atom each represents.

Key Examples You Must Know

NaCl — 1 electron transferred from Na to Cl.
MgO — 2 electrons transferred from Mg to O.
MgCl2 — 2 electrons transferred from Mg, one to each of 2 Cl atoms.
Na2O — 2 Na atoms each transfer 1 electron to 1 O atom.
CaCl2 — 2 electrons transferred from Ca, one to each of 2 Cl atoms.

Exam Tip: When drawing dot-and-cross diagrams for ionic bonding, always include the square brackets and the charge. If you forget the charge notation (e.g. [Na]+ and [Cl]-), you will lose marks even if the rest of the diagram is correct.

Formulae of Ionic Compounds

The formula of an ionic compound must balance so that the overall charge is zero. This is because the total positive charge from the cations must equal the total negative charge from the anions.

Compound	Cation	Anion	Formula	Explanation
Sodium chloride	Na+	Cl-	NaCl	1+ and 1- balance
Magnesium oxide	Mg2+	O2-	MgO	2+ and 2- balance
Magnesium chloride	Mg2+	Cl-	MgCl2	2+ needs two 1- ions
Sodium oxide	Na+	O2-	Na2O	Two 1+ ions balance 2-
Calcium chloride	Ca2+	Cl-	CaCl2	2+ needs two 1- ions
Aluminium oxide	Al3+	O2-	Al2O3	Two 3+ balance three 2-

Why Does Ionic Bonding Happen?

Ionic bonding occurs because the resulting ionic compound is more energetically stable than the individual atoms. The process releases energy overall, as the electrostatic attraction between the oppositely charged ions releases a large amount of energy (known as lattice energy). The atoms achieve the stable electron configuration of a noble gas, which is the lowest energy state for their electrons.

Common Mistakes

Saying that ionic bonding involves sharing electrons — it does not. Ionic bonding involves the transfer of electrons.
Forgetting that the electrons move, not the protons. The nucleus of each atom does not change.
Writing the formula incorrectly — always check that the charges balance to zero.
Drawing dot-and-cross diagrams without square brackets or charges.
Confusing ions (charged particles) with atoms (neutral particles).

Summary

Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions.
Ionic bonds form when metals transfer electrons to non-metals.
Metal atoms lose electrons to form positive ions (cations).
Non-metal atoms gain electrons to form negative ions (anions).
Both ions achieve a full outer shell (noble gas configuration).
Dot-and-cross diagrams show the transfer of electrons, with square brackets and charges around each ion.
The formula of an ionic compound must have balanced charges (overall charge of zero).
Key examples include NaCl, MgO, MgCl2, Na2O, and CaCl2.

Exam Tip: A 6-mark question may ask you to "describe and explain the formation of ionic bonds in sodium chloride." Structure your answer clearly: (1) state what ionic bonding is, (2) describe the electron configurations of Na and Cl, (3) explain the electron transfer, (4) state the charges on the resulting ions, (5) explain the electrostatic attraction. Use a dot-and-cross diagram to support your answer if asked.