Radioactivity and half-lifeIB MYP Chemistry: Mind map

Exam questions on Radioactivity and half-life

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.
   Explain why the alpha source inside the detector is safe for people in the room.2 marks
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.
   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
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 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