Ionic Bonding

Ionic bonding is the transfer of electrons from a metal atom to a non-metal atom, forming charged particles called ions. These oppositely charged ions are held together by strong electrostatic forces of attraction. Understanding ionic bonding requires you to know electron configurations, draw dot-and-cross diagrams, and explain why atoms form ions.

Why Do Atoms Form Ions?

Atoms are most stable when they have a full outer shell of electrons (like a noble gas). Metals have a few electrons in their outer shell and find it easier to lose them. Non-metals are close to a full outer shell and find it easier to gain electrons.

Atom Type	Outer Shell Electrons	Action	Ion Formed	Charge
Metal (Group 1)	1	Loses 1 electron	Positive ion (+1)	e.g. Na⁺
Metal (Group 2)	2	Loses 2 electrons	Positive ion (+2)	e.g. Mg²⁺
Non-metal (Group 6)	6	Gains 2 electrons	Negative ion (−2)	e.g. O²⁻
Non-metal (Group 7)	7	Gains 1 electron	Negative ion (−1)	e.g. Cl⁻

Metal atoms lose electrons → positive ions (cations)

Non-metal atoms gain electrons → negative ions (anions)

flowchart LR
    M["Metal atom\n(e.g. Na: 2,8,1)"] -->|"Loses outer\nelectron(s)"| MI["Metal ion\n(e.g. Na⁺: 2,8)"]
    NM["Non-metal atom\n(e.g. Cl: 2,8,7)"] -->|"Gains\nelectron(s)"| NMI["Non-metal ion\n(e.g. Cl⁻: 2,8,8)"]
    MI -->|"Electrostatic\nattraction"| BOND["Ionic bond"]
    NMI -->|"Electrostatic\nattraction"| BOND

Electron Configurations of Common Ions

Atom	Electron Config	Ion	Ion Electron Config	Same Config As
Na (11)	2,8,1	Na⁺	2,8	Neon
Mg (12)	2,8,2	Mg²⁺	2,8	Neon
Al (13)	2,8,3	Al³⁺	2,8	Neon
O (8)	2,6	O²⁻	2,8	Neon
Cl (17)	2,8,7	Cl⁻	2,8,8	Argon
K (19)	2,8,8,1	K⁺	2,8,8	Argon
Ca (20)	2,8,8,2	Ca²⁺	2,8,8	Argon

The ions have the same electron configuration as the nearest noble gas — this is why they are stable.

Dot-and-Cross Diagrams

In a dot-and-cross diagram:

Show only the outer shell electrons of each atom
Use dots (•) for one atom and crosses (×) for the other
Show the transfer of electrons
Draw square brackets around each ion with the charge outside

Worked Example 1 — Sodium Chloride (NaCl)

Step 1: Na has 1 outer electron (dot). Cl has 7 outer electrons (crosses).

Step 2: Na transfers its 1 electron to Cl.

Step 3: Na becomes Na⁺ (2,8). Cl becomes Cl⁻ (2,8,8).

Step 4: Draw both ions in square brackets with charges.

The electron that was transferred is shown as a dot in the chloride ion's outer shell to show where it came from.

Worked Example 2 — Magnesium Oxide (MgO)

Step 1: Mg has 2 outer electrons. O has 6 outer electrons.

Step 2: Mg transfers both electrons to O.

Step 3: Mg becomes Mg²⁺ (2,8). O becomes O²⁻ (2,8).

One Mg provides exactly the 2 electrons that one O needs, so the ratio is 1:1.

Worked Example 3 — Magnesium Chloride (MgCl₂)

Step 1: Mg has 2 electrons to lose. Each Cl needs 1 electron.

Step 2: Mg transfers 1 electron to each of 2 Cl atoms.

Step 3: Mg becomes Mg²⁺. Each Cl becomes Cl⁻.

The ratio is 1 Mg : 2 Cl, giving the formula MgCl₂.

Worked Example 4 — Calcium Fluoride (CaF₂)

Ca (2,8,8,2) loses 2 electrons → Ca²⁺

Each F (2,7) gains 1 electron → F⁻

Ratio: 1 Ca : 2 F → CaF₂

Working Out the Formula from Charges

The total positive charge must equal the total negative charge (the compound is neutral overall).

Metal Ion	Non-metal Ion	Formula	Reasoning
Na⁺	Cl⁻	NaCl	+1 and −1 balance
Mg²⁺	Cl⁻	MgCl₂	+2 needs 2 × (−1)
Na⁺	O²⁻	Na₂O	2 × (+1) balances −2
Ca²⁺	O²⁻	CaO	+2 and −2 balance
Al³⁺	O²⁻	Al₂O₃	2×(+3) = 3×(−2) = 6
Mg²⁺	O²⁻	MgO	+2 and −2 balance

Quick method: Cross the charges to get the subscripts. For Al³⁺ and O²⁻: Al₂O₃ (the 3 from Al goes to O; the 2 from O goes to Al).

The Nature of the Ionic Bond

An ionic bond is the strong electrostatic force of attraction between oppositely charged ions.

This definition must be precise in the exam. Key points:

It is electrostatic (between charges)
It is between oppositely charged ions
It is strong

The transfer of electrons creates the ions; the attraction between the ions IS the bond.

Worked Example 5 — Writing the Ionic Equation

Question: Write the ionic equation for the formation of sodium chloride from its atoms.

Solution:

Na → Na⁺ + e⁻ (sodium loses one electron)

Cl + e⁻ → Cl⁻ (chlorine gains one electron)

Overall: Na + Cl → Na⁺ + Cl⁻ → NaCl

Worked Example 6 — Predicting the Formula of an Unknown Compound

Question: Lithium (Group 1) reacts with nitrogen (Group 5). Predict the formula of the compound.

Solution:

Li is in Group 1 → loses 1 electron → Li⁺

N is in Group 5 → has 5 outer electrons → needs 3 more → gains 3 electrons → N³⁻

To balance charges: 3 × Li⁺ (total +3) balances 1 × N³⁻ (total −3)

Formula: Li₃N (lithium nitride)

Ionic Bonding and the Periodic Table

flowchart LR
    G1["Group 1 metals\nLose 1 e⁻\nForm +1 ions"] --> IONIC["Ionic\nCompounds"]
    G2["Group 2 metals\nLose 2 e⁻\nForm +2 ions"] --> IONIC
    G6["Group 6 non-metals\nGain 2 e⁻\nForm −2 ions"] --> IONIC
    G7["Group 7 non-metals\nGain 1 e⁻\nForm −1 ions"] --> IONIC

The position of an element in the periodic table tells you exactly what ion it will form:

Group number = number of outer electrons (for main groups)
Metals lose their outer electrons to form positive ions
Non-metals gain electrons to fill their outer shell and form negative ions
The charge on the ion = group number (for metals) or 8 − group number (for non-metals)

Common GCSE Exam Mistakes

Mistake	Correction
Saying atoms "share" electrons in ionic bonding	Ionic bonding involves TRANSFER, not sharing
Drawing molecules for ionic compounds	Ionic compounds form giant lattices, not molecules
Forgetting to show the charge on each ion	Always write the charge outside the square brackets
Getting the charge wrong	Group 1 metals → +1, Group 2 → +2. Group 6 non-metals → −2, Group 7 → −1
Saying "sodium chloride molecule"	Say "formula unit" — there are no molecules in ionic compounds
Not showing where the transferred electron ends up	The dot-and-cross diagram must show the transferred electron in the receiving ion
Forgetting that ions have the electronic structure of a noble gas	After transfer, both ions should have full outer shells

Key Points

Ionic bonding is the transfer of electrons from metals to non-metals, forming positive and negative ions.
Ions have full outer shells and the electron configuration of a noble gas.
The ionic bond is a strong electrostatic attraction between oppositely charged ions.
Dot-and-cross diagrams show the outer electrons only, with dots for one atom and crosses for the other.
The formula of an ionic compound is determined by balancing the charges so the compound is neutral.
Use the group number to predict the charge on an ion: Group 1 → +1, Group 2 → +2, Group 6 → −2, Group 7 → −1.