The questions are on the atoms and molecules test. Take it under real conditions before reading the mark scheme.
Quick answer: An atom is the smallest particle of an element, and a molecule is two or more atoms bonded together. This mark scheme gives the model answers and the marks for the 8 questions of the grade 8 science test on atoms and molecules, from definitions to ethane, with the equivalent answers to accept.
The model answers below show full marks. Follow the marking notes to see where each mark is earned, and note that a correct answer without a method may earn fewer marks.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-1
Question 1 Definitions (2 marks)
(a) An atom is the smallest particle of an element. (b) A molecule is two or more atoms bonded together.
Marking: 1 mark for each correct definition.
Question 2 Counting atoms (2 marks)
(a) The coefficient is 4, so 4 molecules. (b) Each molecule has 1 + 3 = 4 atoms, so 4 × 4 = 16 atoms.
Marking: 1 mark for 4, 1 mark for 16.
Question 3 Model Y (3 marks)
(a) Hydrogen and oxygen. (b) 2 H and 2 O: H₂O₂. (c) The sticks join H–O, O–O and O–H: 3 bonds.
Marking: 1 mark for each part.
Question 4 Element or compound (3 marks)
(a) Both are molecules, made of bonded atoms, and both contain oxygen atoms. (b) O₂ contains only one kind of atom, so it is an element. CO₂ contains two different kinds of atom (carbon and oxygen), so it is a compound.
Marking: 1 mark for (a); 1 mark for stating the difference and 1 for explaining it.
Question 5 Counting atoms in a line (2 marks)
Number = 0.050 ÷ 1 × 10⁻¹⁰ = 5.0 × 10⁸ atoms.
Marking: 1 mark for correct division, 1 mark for the answer.
Question 6 Spot the mistake (2 marks)
The coefficient 2 multiplies the whole formula, so there are two molecules. Each has 2 H and 1 O, giving 4 hydrogen atoms and 2 oxygen atoms.
Marking: 1 mark for explaining the coefficient, 1 mark for the correct numbers.
Question 7 Value of models (3 marks)
It shows how many atoms there are and how they are connected. It is unlike a real molecule because real atoms have no colour and are not solid balls, and the sticks and the sizes shown are not real.
Marking: 1 mark for the strength, 1 mark for each limitation.
Question 8 Ethane (3 marks)
(a) 2 + 6 = 8 atoms. (b) 6 ÷ 8 × 100 = 75 %.
Marking: 1 mark for 8; 1 mark for the method 6/8; 1 mark for 75 %.
