Ionic Bonding

Q: Do I need to memorize transition metal and polyatomic ion charges?

Yes, charges for , , , , , and the listed polyatomic ions are not provided on the periodic table and must be recalled for exams.

Edexcel International GCSE Chemistry· 1.37–1.43· 18 min read

1. Ion Formation & Common Ion Charges★★☆☆☆⏱ 4 min

Ions form when atoms lose or gain electrons to achieve a full outer electron shell (stable noble gas configuration). Metals lose electrons to form positively charged cations, while non-metals gain electrons to form negatively charged anions.

📘 Definition

Ion

A charged particle formed by the loss or gain of one or more electrons from an atom

For main group elements, ion charge can be predicted from their group number: Group 1 → +1, Group 2 → +2, Group 3 → +3, Group 5 → -3, Group 6 → -2, Group 7 → -1. Transition metal charges ( $A g^{+}$ , $C u^{2 +}$ , $F e^{2 +}$ , $F e^{3 +}$ , $P b^{2 +}$ , $Z n^{2 +}$ ) and polyatomic ion charges ( $H^{+}$ , $O H^{-}$ , $N H_{4}^{+}$ , $C O_{3}^{2 -}$ , $N O_{3}^{-}$ , $S O_{4}^{2 -}$ ) must be memorized.

📐 Worked Example

Predict the charge of ions formed by (a) magnesium (Group 2), (b) sulfur (Group 6), (c) silver.

1
Group 2 metals lose 2 outer electrons to form a full outer shell, so the magnesium ion has a +2 charge: $M g^{2 +}$
2
Group 6 non-metals gain 2 electrons to form a full outer shell, so the sulfide ion has a -2 charge: $S^{2 -}$
3
Silver ion charge is a required recall value, so the silver ion has a +1 charge: $A g^{+}$

✓ Quick check

What is the charge of an aluminium ion (Group 3)?
Reveal answer
+3 —
Group 3 metals lose 3 outer electrons to form 3+ ions.
What is the charge of a carbonate ion?
Reveal answer
-2 —
Carbonate is a polyatomic ion with a fixed 2- charge that must be memorized.

2. Writing Charge-Neutral Ionic Formulae★★★☆☆⏱ 5 min

Ionic compounds are electrically neutral overall, so the total positive charge from cations must equal the total negative charge from anions. Use multipliers to balance charges, and wrap polyatomic ions in brackets if you need more than one copy of the ion in the formula.

📘 Definition

Polyatomic ion

A charged particle made of two or more covalently bonded atoms

📐 Worked Example

Write the formula for (a) aluminium oxide, (b) ammonium sulfate.

1
For aluminium oxide: Identify ion charges: $A l^{3 +}$ (Group 3) and $O^{2 -}$ (Group 6). The lowest common multiple of 3 and 2 is 6, so you need 2 $A l^{3 +}$ ions (total +6 charge) and 3 $O^{2 -}$ ions (total -6 charge). The formula is $A l_{2} O_{3}$ .
2
For ammonium sulfate: Identify ion charges: $N H_{4}^{+}$ (polyatomic) and $S O_{4}^{2 -}$ (polyatomic). You need 2 $N H_{4}^{+}$ ions to balance the 2- charge of 1 sulfate ion. Wrap ammonium in brackets because it is a polyatomic ion with a multiplier. The formula is $(N H_{4})_{2} S O_{4}$ .

3. Dot-and-Cross Diagrams for Ionic Compounds★★★☆☆⏱ 5 min

Dot-and-cross diagrams show electron transfer between metals and non-metals to form ionic compounds. Only outer electron shells are drawn, with electrons from one element shown as dots and electrons from the other as crosses. Each resulting ion is enclosed in square brackets, with its overall charge written at the top right outside the brackets.

📘 Definition

Ionic dot-and-cross diagram

A diagram showing the transfer of outer electrons between atoms to form oppositely charged ions, with electrons from different atoms distinguished by dots and crosses

📐 Worked Example

Draw a dot-and-cross diagram for the formation of magnesium chloride ( $M g C l_{2}$ ).

1
Draw the outer electron shells of 1 magnesium atom (2 outer electrons, drawn as dots) and 2 chlorine atoms (7 outer electrons each, drawn as crosses).
2
Show the magnesium atom losing both outer electrons, one transferred to each chlorine atom.
3
Draw the resulting ions: $M g^{2 +}$ with an empty outer shell in square brackets, charge +2 outside. Each $C l^{-}$ ion has a full outer shell of 8 electrons (7 crosses + 1 dot) in square brackets, charge -1 outside.

4. Ionic Bonding & Giant Ionic Lattice Properties★★☆☆☆⏱ 4 min

Ionic bonding is the strong electrostatic attraction between oppositely charged ions, which holds ions together in a regular 3D structure called a giant ionic lattice.

📘 Definition

Giant ionic lattice

A regular three-dimensional arrangement of oppositely charged ions held together by strong ionic bonds

Giant ionic lattices have high melting and boiling points because the strong electrostatic forces between oppositely charged ions require a large amount of heat energy to overcome. Solid ionic compounds do not conduct electricity, as ions are fixed in position in the lattice and cannot move to carry charge. When molten (melted) or dissolved in water (aqueous solution), ions are free to move, so the compound conducts electricity.

📐 Worked Example

Explain why sodium chloride has a melting point of 801°C.

1
Sodium chloride is a giant ionic lattice made of $N a^{+}$ and $C l^{-}$ ions.
2
Strong electrostatic forces of attraction exist between the oppositely charged ions.
3
A large amount of heat energy is required to overcome these strong forces, resulting in a high melting point.

5. Common Pitfalls

Wrong move:

Forgetting brackets around polyatomic ions with multipliers (e.g. writing $N H_{4} 2 S O_{4}$ instead of $(N H_{4})_{2} S O_{4}$ )

Why:

Omitting brackets incorrectly suggests only the last atom in the polyatomic ion is multiplied, leading to an invalid formula.

Correct move:

Always wrap polyatomic ions in brackets if you need more than one copy of the ion to balance charge.

Wrong move:

Drawing dot-and-cross diagrams without square brackets and charge labels around ions

Why:

This fails to show that electrons are transferred (not shared) and that the resulting particles are charged.

Correct move:

Enclose every ion in square brackets, with the overall charge written at the top right outside the brackets.

Wrong move:

Stating solid ionic compounds conduct electricity

Why:

Ions are fixed in position in the solid giant lattice and cannot move to carry charge.

Correct move:

Only state ionic compounds conduct electricity when molten or dissolved in water, as ions are free to move in these states.

Wrong move:

Changing ion charges to balance formulae instead of adjusting ion counts

Why:

Ion charges are fixed for a given ion and cannot be altered.

Correct move:

Adjust the number of each ion until the total positive charge equals the total negative charge.

Wrong move:

Using an incorrect charge for transition metal or polyatomic ions

Why:

These charges are not derivable from the periodic table and must be recalled.

Correct move:

Memorize all required ion charges listed in the specification to avoid this common error.

6. Quick Reference Cheatsheet

Ion Category	Ion Examples	Charge	Example Formula
Group 1 metals	$L i^{+}, N a^{+}, K^{+}$	+1	$N a C l$
Group 2 metals	$M g^{2 +}, C a^{2 +}$	+2	$M g O$
Group 3 metals	$A l^{3 +}$	+3	$A l_{2} O_{3}$
Group 7 non-metals	$F^{-}, C l^{-}, B r^{-}$	-1	$K B r$
Group 6 non-metals	$O^{2 -}, S^{2 -}$	-2	$N a_{2} S$
Group 5 non-metals	$N^{3 -}$	-3	$M g_{3} N_{2}$
Required transition metals	$A g^{+}, C u^{2 +}, F e^{2 +}, F e^{3 +}, P b^{2 +}, Z n^{2 +}$	Varies (recall)	$C u S O_{4}$
Required polyatomic ions	$H^{+}, O H^{-}, N H_{4}^{+}, C O_{3}^{2 -}, N O_{3}^{-}, S O_{4}^{2 -}$	Varies (recall)	$(N H_{4})_{2} S O_{4}$

7. Frequently Asked

Do I need to memorize transition metal and polyatomic ion charges?

Yes, charges for $A g^{+}$ , $C u^{2 +}$ , $F e^{2 +}$ , $F e^{3 +}$ , $P b^{2 +}$ , $Z n^{2 +}$ and the listed polyatomic ions are not provided on the periodic table and must be recalled for exams.

Do I need to draw inner electron shells for dot-and-cross diagrams?

No, the Edexcel specification only requires you to draw outer electron shells for all atoms in ionic dot-and-cross diagrams.

What's Next

Now that you have mastered core ionic bonding concepts, you are ready to progress to related topics in the Principles of Chemistry unit. Next, you will study covalent bonding, which forms when non-metal atoms share electrons rather than transfer them, and learn to compare the properties of ionic and simple covalent compounds. Following that, you will cover metallic bonding, the third main type of chemical bonding, before moving to electrolysis, where you will apply your knowledge of ionic compound conductivity to explain electrolytic reactions. These topics are frequently tested together in structured exam questions, so ensure you are confident with ionic formula writing and dot-and-cross diagrams before proceeding, as they form the foundation for all bonding content in the course.