How do you write the formula of an ionic compound?

Write each ion with its charge, balance the charges so the total is zero, and give the ratio without charges. 1. Write the symbol and charge of each ion. 2. Balance the charges so the total is zero. 3. The ratio gives the formula; do not include the charges.

Ionic Bonding and Ionic Compounds Notes

AQA GCSE Chemistry: Revision notes

Key facts

Ionic bonding is metal plus non-metal: electrons are transferred, and oppositely charged ions attract.
The metal loses electrons to form a cation (positive); the non-metal gains electrons to form an anion (negative).
Both ions end up with a full outer shell.
Ionic compounds form a giant ionic lattice, not individual molecules.
High melting points; conduct only when molten or dissolved, because the ions must be free to move.

Na atom

2,8,1

Na⁺ ion

2,8

Cl atom

2,8,7

Cl⁻ ion

2,8,8

Sodium loses an electron to chlorine, forming Na⁺ and Cl⁻.

What is ionic bonding?

Ionic bonding is the electrostatic attraction between oppositely charged ions, formed when a metal transfers electrons to a non-metal.

A metal loses electrons to become a cation (positive ion). A non-metal gains them to become an anion (negative ion). Both end up with a full outer shell, a stable noble gas arrangement.

The opposite charges attract, and this electrostatic attraction is the ionic bond. There are no individual molecules, just a giant lattice of ions.

1
Sodium atom
has 1 outer electron
2
Electron transferred
from sodium to chlorine
3
Ions formed
$\ce{Na+}$ and $\ce{Cl-}$ , both with full outer shells
4
Ionic bond
electrostatic attraction between the opposite charges

Forming sodium chloride.

What is the ionic bond?

Working out ion charges

The charge tells you how many electrons were transferred. For metals it matches the group number; for non-metals count back from 8.

Group 2 metals always form 2+ ions. For non-metals, count back from Group 8: Group 7 is 1 away, so 1−; Group 6 is 2 away, so 2−.

Magnesium loses 2 electrons ( $\ce{Mg^2+}$ , 2,8) and oxygen gains 2 ( $\ce{O^2-}$ , 2,8).

\ce{Na+}

	Group 1 metals	Group 2 metals	Group 6 non-metals	Group 7 non-metals
Action	Lose 1 electron	Lose 2 electrons	Gain 2 electrons	Gain 1 electron
Charge	+1	+2	−2	−1
Examples	$\ce{Na+}$ , $\ce{K+}$ , $\ce{Li+}$	$\ce{Mg^2+}$ , $\ce{Ca^2+}$	$\ce{O^2-}$ , $\ce{S^2-}$	$\ce{Cl-}$ , $\ce{Br-}$ , $\ce{F-}$

Group 1 metals

Action:: Lose 1 electron
Charge:: +1
Examples:: $\ce{Na+}$ , $\ce{K+}$ , $\ce{Li+}$

Group 2 metals

Action:: Lose 2 electrons
Charge:: +2
Examples:: $\ce{Mg^2+}$ , $\ce{Ca^2+}$

Group 6 non-metals

Action:: Gain 2 electrons
Charge:: −2
Examples:: $\ce{O^2-}$ , $\ce{S^2-}$

Group 7 non-metals

Action:: Gain 1 electron
Charge:: −1
Examples:: $\ce{Cl-}$ , $\ce{Br-}$ , $\ce{F-}$

Mg atom

2,8,2

Mg²⁺ ion

2,8

O atom

2,6

O²⁻ ion

2,8

Magnesium loses 2 electrons; oxygen gains 2.

What ion does a Group 6 non-metal such as oxygen form?

Writing ionic formulae

Write each ion with its charge, balance the charges so the total is zero, and give the ratio without charges.

Write the symbol and charge of each ion.
Balance the charges so the total is zero.
The ratio gives the formula; do not include the charges.

Never assume one of each ion: writing $\ce{MgCl}$ instead of $\ce{MgCl2}$ is wrong.

Mg²⁺

2,8

Cl⁻

2,8,8

Cl⁻

2,8,8

Magnesium chloride: one Mg²⁺ (2+) is balanced by two Cl⁻ (2 × 1−), so the formula is MgCl₂.

\ce{Mg^2+}

	Magnesium chloride	Aluminium oxide
Ions	$\ce{Mg^2+}$ , $\ce{Cl-}$	$\ce{Al^3+}$ , $\ce{O^2-}$
Balancing	2 × $\ce{Cl-}$ cancels 1 × $\ce{Mg^2+}$	2 × 3+ = 6+ and 3 × 2− = 6−
Formula	$\ce{MgCl2}$	$\ce{Al2O3}$

Magnesium chloride

Ions:: $\ce{Mg^2+}$ , $\ce{Cl-}$
Balancing:: 2 × $\ce{Cl-}$ cancels 1 × $\ce{Mg^2+}$
Formula:: $\ce{MgCl2}$

Aluminium oxide

Ions:: $\ce{Al^3+}$ , $\ce{O^2-}$
Balancing:: 2 × 3+ = 6+ and 3 × 2− = 6−
Formula:: $\ce{Al2O3}$

Worked example

Write the formula of magnesium chloride.

1
Write the ions: $\ce{Mg^2+}$ and $\ce{Cl-}$ .
2
Balance the charges: 2 × $\ce{Cl-}$ cancels 1 × $\ce{Mg^2+}$ (2+ and 2− = zero).

What is the formula of sodium oxide?

The giant ionic lattice

All ionic compounds form a giant ionic lattice: a regular, repeating 3D arrangement of alternating positive and negative ions.

The giant ionic lattice has millions of ions held by strong electrostatic forces in all directions. There are no individual molecules, and the pattern repeats identically in three dimensions.

In $\ce{NaCl}$ , each $\ce{Na+}$ is surrounded by 6 $\ce{Cl-}$ ions and each $\ce{Cl-}$ by 6 $\ce{Na+}$ ions.

Na⁺

2,8

Cl⁻

2,8,8

The ions in sodium chloride: in the lattice each Na⁺ is surrounded by 6 Cl⁻ and each Cl⁻ by 6 Na⁺.

Each $\ce{Na+}$ 6 $\ce{Cl-}$ neighbours
Each $\ce{Cl-}$ 6 $\ce{Na+}$ neighbours

How many $\ce{Cl-}$ ions surround each $\ce{Na+}$ ion in sodium chloride?

Properties of ionic compounds

High melting points because of strong forces throughout the lattice; they conduct only when molten or dissolved because their ions must be free to move.

Say strong electrostatic forces between oppositely charged ions throughout the giant ionic lattice for melting point answers: never just 'strong bonds'. Higher ion charges mean stronger forces and a higher melting point.

For conduction, say the ions are free to move. Ionic compounds conduct via ion movement, not electron movement: free electrons are a feature of metals.

Melting points of two ionic compounds: higher ion charges (Mg²⁺, O²⁻) mean stronger forces and a higher melting point.

Behaviour Reason Melting point Very high Strong forces across the whole lattice need huge energy to break Conducts as a solid No Ions are fixed in the lattice and cannot move Conducts molten or dissolved Yes Ions are free to move and carry charge Dissolves in water Many do Water molecules pull ions away from the lattice

	Behaviour	Reason
Melting point	Very high	Strong forces across the whole lattice need huge energy to break
Conducts as a solid	No	Ions are fixed in the lattice and cannot move
Conducts molten or dissolved	Yes	Ions are free to move and carry charge
Dissolves in water	Many do	Water molecules pull ions away from the lattice

Behaviour

Melting point:: Very high
Conducts as a solid:: No
Conducts molten or dissolved:: Yes
Dissolves in water:: Many do

Reason

Melting point:: Strong forces across the whole lattice need huge energy to break
Conducts as a solid:: Ions are fixed in the lattice and cannot move
Conducts molten or dissolved:: Ions are free to move and carry charge
Dissolves in water:: Water molecules pull ions away from the lattice

Why does solid sodium chloride not conduct electricity?

Try an exam question

Describe, in terms of electrons, what happens when sodium reacts with chlorine to form sodium chloride. Name the type of bonding in sodium chloride.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.