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Auralith Learning

Year 8 · Science · Elements, compounds and the periodic table · Lesson 2

Compounds, mixtures and formulae

What compounds and mixtures are, how to tell them apart in particle diagrams, counting atoms in chemical formulae, and naming compounds.

Whiteboard video lesson: Compounds, mixtures and formulae

27-minute video lesson

Teaching, worked examples and 20 practice questions solved step by step.

Watch free for a month

What your child will learn

  • Describe compounds and mixtures
  • Identify elements, compounds and mixtures in particle diagrams
  • Count the atoms in a chemical formula
  • Name simple compounds using -ide and -ate
  • Explain why compounds have different properties from their elements

The method

  1. A compound is two or more different elements chemically joined, with new properties.
  2. A mixture is substances not chemically joined, so they're easy to separate.
  3. In a formula, the small number counts the atom just before it; no number means 1.
  4. Naming: metal first; -ide for two elements; -ate when oxygen is also present.

Key words: compound, mixture, formula, chemically joined, -ide, -ate, molecule.

Worked examples

The first three practice questions from the lesson, with every step.

Question 1

How many atoms of each element are in CO₂ (carbon dioxide)? How many atoms in total?

  1. Step 1: Elements: carbon, oxygen
  2. Step 2: 1 C, 2 O
  3. Step 3: Total: 1 + 2 = 3

Answer: 1 C, 2 O; 3 atoms in total

Question 2

How many atoms of each element are in NH₃ (ammonia)? How many atoms in total?

  1. Step 1: Elements: nitrogen, hydrogen
  2. Step 2: 1 N, 3 H
  3. Step 3: Total: 1 + 3 = 4

Answer: 1 N, 3 H; 4 atoms in total

Question 3

How many atoms of each element are in CH₄ (methane)? How many atoms in total?

  1. Step 1: Elements: carbon, hydrogen
  2. Step 2: 1 C, 4 H
  3. Step 3: Total: 1 + 4 = 5

Answer: 1 C, 4 H; 5 atoms in total