Radioactivity and half-lifeIB MYP Chemistry: Subtopic test

1. 1
   A smoke detector in a school contains a tiny sample of americium-241, which emits alpha particles. The alpha particles ionise the air between two metal plates, so a small current flows. When smoke enters the detector, the current falls and the alarm sounds.

   (a)

   What is an alpha particle?

   [1 mark]

   • AAn electron from the nucleus
   • BA high-energy photon
   • CA neutron
   • DTwo protons and two neutrons

   (b)

   Which property of alpha particles makes them suitable for ionising the air in the detector?

   [1 mark]

   • AThey are strongly ionising
   • BThey are very penetrating
   • CThey have no charge
   • DThey are electromagnetic waves

   (c)

   Explain why the alpha source inside the detector is safe for people in the room.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A nucleus of uranium-238, which has atomic number 92, is unstable and decays by emitting an alpha particle. The new nucleus is an atom of thorium.

   (a)

   What is the mass number of the thorium nucleus formed?

   [1 mark]

   • A238
   • B236
   • C234
   • D242

   (b)

   What is the atomic number of the thorium nucleus formed?

   [1 mark]

   • A94
   • B90
   • C88
   • D92

   (c)

   The thorium nucleus, with mass number 234 and atomic number 90, then decays by beta emission. Give the mass number and atomic number of the nucleus formed, and explain why the atomic number changes.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A hospital in Dubai uses the radioactive tracer technetium-99m to check how well a patient's kidneys work. The tracer emits gamma rays and has a half-life of 6 hours. It is injected into the patient, and a detector outside the body picks up the gamma rays.

   (a)

   A patient is given a dose of the tracer with an activity of 800 counts per minute. Calculate the activity of the tracer after 18 hours.

   [3 marks]

   (b)

   Explain why a gamma emitter with a short half-life is a good choice for this tracer, and why an alpha emitter would not be suitable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An archaeologist at a university in Cairo finds a wooden bowl and dates it using carbon-14, which has a half-life of 5700 years. Living wood has a carbon-14 activity of 16 counts per minute per gram of carbon. The bowl gives 2 counts per minute per gram, after correcting for background radiation. The university's radiation laboratory also supplies sealed gamma sources to factories that sterilise medical equipment and some foods.

   (a)

   Calculate the age of the bowl, showing your working, and explain why measuring carbon-14 activity allows wood to be dated, and state one assumption or limitation of the method.

   [6 marks]

   (b)

   Discuss and evaluate the use of gamma radiation to sterilise medical equipment and food, referring to the hazards of handling radioactive materials.

   [6 marks]

   Total for question 4: 12 marks

End of questions