Howardites

The answer sheet for the worksheet Half-life and dating meteorites. Each answer comes with the reason, so it can be used to mark or to go over the questions with the class.

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

Answers: Half-life and dating meteorites

  1. C: They were the first solids around the young SunThe dated grains are the first solids to form around the young Sun, so their age marks its beginning; when a meteorite landed is a different question.
  2. C: 1/817,190 ÷ 5,730 = 3 half-lives, so (1/2)³ = 1/8 remains.
  3. D: A different elementRadioactive decay changes the nucleus, so the atom becomes a different element.
  4. B: 4 billion years1/16 = (1/2)⁴, so four half-lives have passed: 4 × 1 billion = 4 billion years.
  5. D: 100 counts per minute6 ÷ 2 = 3 half-lives, so the activity halves three times: 800 → 400 → 200 → 100.
  6. False: the right answer is When it will decay is randomThe moment any one nucleus decays cannot be predicted; only the behaviour of a large sample is predictable.
  7. True (When the mineral crystallises)Crystallising traps parent isotopes; from then on daughter isotopes build up inside the mineral.
  8. Far too little of it would have decayed to measure (accept any answer with the same meaning)With a half-life of about 4.5 billion years, a few hundred years would change the amount of uranium-238 by far too little to detect.
  9. A curve that falls steeply, then ever more gently (accept any answer with the same meaning)The number halves in each half-life, so the fall is large at first and smaller and smaller after that.
  10. Lead isotopes from meteorites (accept any answer with the same meaning)Patterson measured lead isotopes in meteorites, including iron sulfide from the meteorite that made Meteor Crater.

Task

Answers will vary. The task comes from the lesson: Model decay with 100 coins or dice. Shake them all, remove those showing heads (or a six), count what is left and repeat for ten throws. Plot the number remaining against throw number and read off the 'half-life' from your graph. Why do the class's combined results give a smoother curve than one person's?

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