Howardites

The answer sheet for the worksheet Isotopes as fingerprints. Each answer comes with the reason, so it can be used to mark or to go over the questions with the class.

Back to the worksheet

GCSE · Ages 14 to 16 (Years 10 and 11)Answer sheet for teachers

Answers: Isotopes as fingerprints

  1. B: Nearly all oxygen atoms are oxygen-16About 99.76% of oxygen atoms are oxygen-16, so the rare heavier isotopes barely change the mean.
  2. C: 8A neutral atom has as many electrons as protons, so all three oxygen isotopes have 8 electrons.
  3. D: 8p + 8n and 8p + 10nIsotopes share the same number of protons (p) but have different numbers of neutrons (n); same mass number is not enough.
  4. B: The differences are tiny, often parts per thousand or lessThe differences between meteorite groups are often a few parts in a thousand or less, so only a sensitive instrument can detect them.
  5. C: 8Every oxygen atom has 8 protons, its atomic number; oxygen-17 differs from oxygen-16 only in having one more neutron.
  6. False: the right answer is 2Both have the same number of protons, so the difference in mass number, 37 − 35 = 2, is all neutrons.
  7. False: the right answer is The one with mass number 7The weighted mean lies closest to the most abundant isotope, and 6.9 is much nearer 7 than 6.
  8. Separates atoms or molecules by their mass (accept any answer with the same meaning)A mass spectrometer sorts atoms or molecules by mass, so it can measure the amounts of each isotope.
  9. It ignores how common each isotope is (accept any answer with the same meaning)A simple mean gives 17, but Aᵣ must weight each isotope by its abundance, giving 16.0.
  10. The Moon (accept any answer with the same meaning)Lunar rocks have almost exactly the same oxygen isotopes as Earth, while Mars and the asteroids have their own signatures.

Task

Answers will vary. The task comes from the lesson: Chlorine is about 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass. Then write three sentences explaining why two rocks with very different minerals can still be shown to come from the same asteroid.

Look for work that uses the key words correctly and gives a reason for each point.

Howardites · teacher copy: keep it separate from the pupil worksheets