Key Learning Objectives
- Distinguish between atoms, molecules, elements, compounds, and mixtures.
- Write and interpret chemical formulas with subscripts (e.g. H₂O, CO₂, CH₄, H₂SO₄).
- Count atoms of each element in complex chemical formulas and balanced equations.
Topic Summary & Core Notes
Master chemical formulas, element symbols, molecular compounds, and the distinction between pure elements, compounds, and mixtures in Year 7 Chemistry.
Key Rules, Formulas & Definitions
Subscript Rule
Subscript number applies ONLY to the element immediately before it: H₂O = 2 Hydrogen, 1 Oxygen
Bracket Multiplier Rule
Subscripts outside brackets multiply everything inside: Ca(OH)₂ = 1 Ca, 2 O, 2 H
Coefficient Rule
Large number in front multiplies the entire molecule: 3 CO₂ = 3 Carbon, 6 Oxygen atoms
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Counting Atoms in a Molecule
Question: Calculate the total number of atoms in two molecules of Sulfuric Acid: 2 H₂SO₄.
Step-by-Step Solution:
- Step 1: Count atoms in one H₂SO₄ molecule: 2 Hydrogen + 1 Sulfur + 4 Oxygen = 7 atoms.
- Step 2: Multiply by coefficient (2): 2 × 7 atoms = 14 total atoms.
Final Answer: 14 atoms (4 H, 2 S, 8 O)
Senior Examiner Insight: Break down each element individually before adding together.
Worked Example 2
Worked Example 2: Identifying Pure Elements vs Compounds
Question: Explain whether Ozone (O₃) is an element or a compound.
Step-by-Step Solution:
- Step 1: Definition of Element: A substance consisting of only ONE type of atom.
- Step 2: Definition of Compound: A substance containing two or more DIFFERENT elements chemically bonded.
- Step 3: Analyze O₃: Ozone contains 3 oxygen atoms, but all are the same element (Oxygen).
Final Answer: Ozone (O₃) is an element because it contains only one type of atom (Oxygen).
Senior Examiner Insight: Do not confuse 'molecule' with 'compound'. O₃ and O₂ are molecules of elements.
Common Pitfalls & Examiner Warnings
Common Mistake: Thinking O₂ or O₃ is a compound
Compounds require TWO OR MORE DIFFERENT elements chemically combined.
Common Mistake: Confusing subscripts with coefficients
2 CO is two molecules of carbon monoxide; CO₂ is one molecule containing two oxygen atoms.
Common Mistake: Forgetting that symbols with lowercase letters belong together
Co is Cobalt (one element); CO is Carbon Monoxide (two elements: Carbon and Oxygen).
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
How many total atoms are present in one formula unit of Calcium Nitrate: Ca(NO₃)₂?
Click to Reveal Answer & Mark Scheme
Correct Answer: C) 9
Detailed Explanation: 1 Calcium + (1 Nitrogen × 2) + (3 Oxygen × 2) = 1 + 2 + 6 = 9 atoms.
Mark Scheme & Scoring Points: 1 mark for 9 atoms.
Question 2 of 3
Which of the following represents a MIXTURE rather than a pure compound?
Click to Reveal Answer & Mark Scheme
Correct Answer: C) Sea water
Detailed Explanation: Sea water contains dissolved salts, water, and minerals that are physically mixed and can be separated by evaporation.
Mark Scheme & Scoring Points: 1 mark for Sea water.
Question 3 of 3
What is the chemical name for the compound CH₄?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Methane
Detailed Explanation: CH₄ is methane, the simplest hydrocarbon alkane.
Mark Scheme & Scoring Points: 1 mark for Methane.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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