New
New
Year 10
AQA
Foundation

Determining ionic formulae

I can deduce the formulae of ionic compounds (including oxides, hydroxides, halides, nitrates, carbonates and sulfates) given the formulae of the constituent ions.

New
New
Year 10
AQA
Foundation

Determining ionic formulae

I can deduce the formulae of ionic compounds (including oxides, hydroxides, halides, nitrates, carbonates and sulfates) given the formulae of the constituent ions.

Lesson details

Key learning points

  1. An empirical formula is the simplest whole number ratio of atoms of each element in a compound.
  2. The formula of an ionic compound is an empirical formula, as there is no definite quantity of each ion.
  3. Ionic compounds ending with "–ide" only contain a metal and a non–metal ion.
  4. Polyatomic ions are ions that are made up of more than one type of atom.
  5. Ionic compounds ending with "–ate" always contain at least three elements, one of which is oxygen.

Common misconception

Using ionic charges to determine the empirical formulae, especially with polyatomic ions. Standard notation is difficult and confusing, e.g. when to use superscripts, subscripts or brackets.

At this stage, pupils will need to just learn the the formulae of the polyatomic ions and accept that a full explanation will come later in their chemistry learning. Provide pupils with plenty of practice of writing and using ionic formulae.

Keywords

  • Empirical formula - The simplest whole number ratio of atoms of each element in a compound.

  • Polyatomic ions - Ions that are made up of more than one type of atom.

  • "–ide" - The name ending for a compound with a metal and non–metal present, for example, potassium iodide (KI).

  • "–ate" - The name ending for a compound with three or more elements present (one of which is oxygen), for example, potassium sulfate (K₂SO₄).

Make learning visible by using ionic chemical jigsaws or different coloured counters/beads when teaching how to calculate the formula of a compound using ionic charge.
Teacher tip

Equipment

None required.

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.
Ions form through the loss or gain of ...
Correct Answer: electrons, electron
Q2.
When a nitrate ion forms, it gains one electron. What charge does a nitrate ion have?
2+
2–
1+
Correct answer: 1–
Q3.
True or false? Non–metal atoms gain electrons to become more stable, negatively charged ions.
Correct answer: true
false
Q4.
Which of the following statements about ammonium nitrate, NH₄NO₃, are correct?
Correct answer: The substance contains three elements.
Correct answer: The formula contains two nitrogen atoms.
The substance contains three oxygen atoms.
The ratio of hydrogen to oxygen in the formula is 3 : 4.
Q5.
Match the following terms to the correct definitions.
Correct Answer:ionic bond,The electrostatic force of attraction between oppositely charged ions.

The electrostatic force of attraction between oppositely charged ions.

Correct Answer:lattice,The regular arrangement of atoms or ions in a 3D space.

The regular arrangement of atoms or ions in a 3D space.

Correct Answer:ion,Atom or molecule with a charge due to the gain or loss of electrons.

Atom or molecule with a charge due to the gain or loss of electrons.

Q6.
Magnesium chloride is an ionic compound. Which of the following statements does not describe magnesium chloride?
It contains Mg²⁺ and Cl⁻ ions.
Correct answer: It forms simple molecules.
It forms a giant lattice.
One Mg²⁺ ion is attracted to two Cl⁻ ions.

6 Questions

Q1.
Match the following key terms to the correct definition.
Correct Answer:empirical formula,Simplest whole number ratio of atoms of each element in a compound.

Simplest whole number ratio of atoms of each element in a compound.

Correct Answer:molecular formula,Shows the actual number of atoms of each element in a compound.

Shows the actual number of atoms of each element in a compound.

Correct Answer:polyatomic ions,Ions made of more than one type of atom.

Ions made of more than one type of atom.

Correct Answer:–ide compounds,Name ending for a compound with a metal and singular non–metal.

Name ending for a compound with a metal and singular non–metal.

Correct Answer:–ate compounds,Name ending for a compound with 3 or more elements, one being oxygen.

Name ending for a compound with 3 or more elements, one being oxygen.

Q2.
The molecular formula for hydrogen peroxide is H₂O₂. What is its empirical formula?
Correct Answer: HO
Q3.
Which of the following are examples of ionic compounds with names ending with –ide?
Correct answer: NaCl
MgSO₄
H₂O
Correct answer: CaO
Q4.
Which of the following are examples of polyatomic ions?
Na⁺
Correct answer: SO₄²⁻
Cl⁻
Cu(NO₃)₂
Correct answer: CO₃⁻
Q5.
The image shows the ionic compound lead(II) chloride. What is the empirical formula for lead(II) chloride?
An image in a quiz
PbCl
Correct answer: PbCl₂
Pb₂Cl
Q6.
The ammonium ion has the formula: NH₄⁺. Sulfate ions have the formula: SO₄²⁻. What is the formula for ammonium sulfate?
NH₄⁺SO₄²⁻
NH₄SO₄
NH₄(SO₄)₂
Correct answer: (NH₄)₂SO₄