The problems are on the atoms and molecules worksheet. Try them on your own first, then compare with the answers below.
Quick answer: An atom is the smallest particle of an element, and a molecule is two or more atoms bonded together. These worked answers correct the 20 problems of the grade 8 science worksheet on atoms and molecules, from water formula to scaling up an atom: each solution shows the formula, the substitution and the final answer with its unit.
Each answer shows the formula or sum used and then the result in bold. Check your counting against the method before you look at the final number.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-1
Problem 1 Water formula
1. Two H and one O: H₂O.2. 2 + 1 = 3 atoms.
3. Hydrogen and oxygen: 2 elements.
Problem 2 Reading subscripts
1. O₂ has 2 atoms, N₂ has 2 atoms, CO₂ has 1 + 2 = 3 atoms: 2, 2 and 3.2. O₂ and N₂ contain only one kind of atom: O₂ and N₂.
Problem 3 Coefficient and subscript
1. The coefficient 3 means 3 molecules.2. The subscript 2 means 2 hydrogen atoms in each molecule.
3. Each molecule has 3 atoms, so 3 × 3 = 9 atoms.
Problem 4 Atom or molecule
1. Helium, written He, is a gas whose particles are single atoms.2. Cl₂, CH₄ and NH₃ are made of bonded atoms, so they are molecules.
3. 1 + 4 = 5 atoms.
Problem 5 True or false
1. False: a molecule contains several atoms, so it is larger than one of its atoms.2. True: the subscript is 2.
3. True, as in H₂O.
4. False: O is one atom, but O₂ is a molecule of two atoms. Statements 2 and 3 are true.
Problem 6 Model X
1. One carbon atom and two oxygen atoms.2. CO₂.
3. 1 + 2 = 3 atoms.
Problem 7 Counting in acid
1. HCl: 1 + 1 = 2. H₂SO₄: 2 + 1 + 4 = 7. NH₃: 1 + 3 = 4.2. H₂SO₄ has the most, with 7 atoms.
Problem 8 Glucose in a drink
1. C = 6, H = 12, O = 6.2. 6 + 12 + 6 = 24 atoms.
3. 12 ÷ 24 = 0.50, so 50 % of the atoms are hydrogen.
Problem 9 Several molecules
1. The coefficient is 4, so 4 molecules.2. C: 4 × 1 = 4. H: 4 × 4 = 16.
3. 4 + 16 = 20 atoms.
Problem 10 Methane model
1. One C and four H: CH₄.2. 4 bonds, one from the carbon to each hydrogen.
3. The colours are a code and the balls and sticks are a simplified picture; real atoms have no colour and no solid sticks joining them.
Problem 11 Ethanol model
1. Carbon: 2. Hydrogen: 6. Oxygen: 1.2. C₂H₆O.
3. The picture has 8 sticks: 8 bonds (9 atoms joined in one connected group).
Problem 12 Size of a molecule
1. 0.28 × 10⁻⁹ m = 2.8 × 10⁻¹⁰ m.2. 1.0 × 10⁻³ ÷ 2.8 × 10⁻¹⁰ = 3.6 × 10⁶, so about 3.6 million molecules.
Problem 13 Spot the errors
1. (i) The subscript is on the wrong atom. (ii) He counted only the oxygen atoms of one molecule; one CO₂ has 3 atoms.2. (i) Water is H₂O. (ii) 3 CO₂ contains 3 × 3 = 9 atoms.
Problem 14 Oxygen and ozone
1. O₂ has 2 atoms and O₃ has 3 atoms.2. The number of atoms in each molecule is different, so the particles are different and they have different properties.
3. 5 × 3 = 15 atoms.
Problem 15 Formula from a description
1. NH₃.2. C₂H₄.
3. CCl₄.
4. NH₃ = 4 atoms, C₂H₄ = 6 atoms, CCl₄ = 5 atoms.
Problem 16 Model kit puzzle
1. Carbon: 18 ÷ 6 = 3. Hydrogen: 30 ÷ 12 = 2.5. Oxygen: 24 ÷ 6 = 4.2. Hydrogen runs out first, giving 2 complete models.
3. Used: C 12, H 24, O 12. Left: 6 carbon, 6 hydrogen, 12 oxygen.
Problem 17 Atom percentages in propane
1. 3 + 8 = 11 atoms.2. 3 ÷ 11 × 100 = 27 % (27.3 %).
3. 8 ÷ 11 × 100 = 73 % (72.7 %).
4. 27.3 + 72.7 = 100.0, so the check works.
Problem 18 Same atoms, different molecules
1. Both have 2 C, 6 H and 1 O, so 9 atoms.2. The atoms are joined in a different order and pattern, so the molecules have different shapes and different properties.
3. They would use the same balls but connect them with sticks in a different arrangement. Same atoms, different arrangement.
Problem 19 Comparing five substances
1. H₂ and N₂ contain only one kind of atom.2. H₂ = 2, H₂O = 3, N₂ = 2, NH₃ = 4, H₂O₂ = 4.
3. NH₃ and H₂O₂ tie with 4 atoms each, so 10 × 4 = 40 atoms.
4. H₂O₂ has one more oxygen atom than H₂O, which makes it a different substance.
Problem 20 Scaling up an atom
1. 0.070 ÷ 1 × 10⁻¹⁰ = 7.0 × 10⁸ times.2. 1.0 × 10⁻⁴ × 7.0 × 10⁸ = 7.0 × 10⁴ m.
3. 7.0 × 10⁴ m ÷ 1000 = 70 km.
