AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Exam Practice: Mass number, atomic number and isotopes
Mass number, atomic number and isotopes · Atomic structure · Lesson 2 of 10 · 13 marks · 14 minutes
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Instructions
• Answer all the questions.
• Write your answers in the spaces provided.
• The marks for each question are shown in brackets - use this as a guide to how much to write.
• The answers are on separate pages at the back. Attempt every question before you look at them.
• Answer all questions. Use the mark allocation as a guide to how much to write.
Question 1 CALCULATE (2 marks)
An atom of carbon-14 is represented as ¹⁴₆C. State the number of protons and the number of neutrons in the atom.
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(Total for Question 1 = 2 marks)
Question 2 EXPLAIN (2 marks)
Explain what is meant by isotopes.
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(Total for Question 2 = 2 marks)
Question 3 COMPARE (3 marks)
Chlorine has two isotopes: ³⁵₁₇Cl and ³⁷₁₇Cl. State one similarity and one difference between atoms of these isotopes.
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(Total for Question 3 = 3 marks)
Question 4 EXPLAIN (3 marks)
A lithium atom ⁷₃Li becomes a positive ion. Explain how this happens and give the number of electrons in the ion if the charge is +1.
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(Total for Question 4 = 3 marks)
Question 5 CALCULATE (3 marks)
An atom of uranium-235 has 92 protons. Calculate the number of neutrons and the number of electrons in a neutral atom of uranium-235.
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(Total for Question 5 = 3 marks)
TOTAL FOR PAPER = 13 MARKS
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Answers and mark scheme
Check your answer only once you have written one.
Question 1 (2 marks)
6 protons; 14 − 6 = 8 neutrons.
• 6 protons 1 mark
• 8 neutrons 1 mark
Question 2 (2 marks)
Atoms of the same element (same number of protons) that have different numbers of neutrons.
• Same number of protons or same element 1 mark
• Different number of neutrons 1 mark
Question 3 (3 marks)
Similarity: same number of protons (17) or electrons. Difference: chlorine-37 has 2 more neutrons (20 rather than 18), so a larger mass number.
• Similarity 1 mark
• Difference in neutrons 1 mark
• With numbers or different mass 1 mark
Question 4 (3 marks)
The atom loses an outer electron. It has 3 protons and 2 electrons, so the charge is +1.
• Loses an electron 1 mark
• 3 protons and 2 electrons 1 mark
• More protons than electrons gives positive charge 1 mark
Question 5 (3 marks)
Neutrons: 235 − 92 = 143. Electrons: 92.
• Neutrons = mass number − atomic number 1 mark
• 143 1 mark
• 92 electrons 1 mark