Quick answer: An atom is the smallest particle of an element, and a molecule is two or more atoms bonded together. This grade 8 science test on atoms and molecules has 8 questions worth 20 marks, to do in 45 minutes: definitions, counting atoms and model Y. The mark scheme is on a separate page; free printable PDF.
This test covers atoms, molecules, formulas and models. Answer every question, show your working for calculations and write units where needed. You have 45 minutes and the test is worth 20 marks.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-1
Question 1 Definitions (2 marks)
Define the terms atom and molecule.- Atom (1 mark)
- Molecule (1 mark)
Question 2 Counting atoms (2 marks)
A chemist writes 4 NH₃.- State the number of molecules. (1 mark)
- Calculate the total number of atoms. (1 mark)
Question 3 Model Y (3 marks)
Look at the model.- State the names of the elements shown. (1 mark)
- Write the formula of the molecule. (1 mark)
- State the number of bonds. (1 mark)
Question 4 Element or compound (3 marks)
Oxygen is O₂ and carbon dioxide is CO₂.- Explain what the two molecules have in common. (1 mark)
- Explain one difference between them. (2 marks)
Question 5 Counting atoms in a line (2 marks)
An atom is about 1 × 10⁻¹⁰ m across. Calculate how many atoms fit side by side along a line 5.0 cm (0.050 m) long.Question 6 Spot the mistake (2 marks)
A student says “2 H₂O means 2 hydrogen atoms and 1 oxygen atom”. Explain why this is wrong and give the correct numbers.Question 7 Value of models (3 marks)
Describe one thing a ball-and-stick model shows well and two ways in which it is unlike a real molecule.Question 8 Ethane (3 marks)
Ethane has the formula C₂H₆.- Calculate the total number of atoms in one molecule. (1 mark)
- Calculate the percentage of atoms that are hydrogen. (2 marks)
