Questions: sources and properties of ionising radiation

What the Activity is for

The diagnostic questions can be used for two main purposes:

• To encourage students to talk and think through the sources and properties of ionising radiation.
• To provide the teacher with formative assessment information about the students' understandings of ionising radiation.

What to Prepare

• printed copies of the diagnostic questions (see below)

What Happens During this Activity

It would be a good idea to get the students to work in pairs on these questions, encouraging each pair to talk through their ideas with each other. Collect responses from all of the pairs and discuss in a whole-class plenary.

Alternatively, the questions might be set for homework prior to the lesson, so that you have time to read through the responses.

The questions review the following aspects of ionising radiation:

Question 1–3: Structure of the atom

Questions 4–5: Sources of radiation

Questions 6–7: Half-life

Question 1: Why does an atom have no overall electric charge?

1. The number of electrons equals the number of neutrons.
2. The number of neutrons equals the number of ions.
3. The number of protons equals the number of electrons.
4. The number of protons equals the number of ions.
5. The number of protons equals the number of neutrons.

Question 1 answer: 3.
Protons carry a single positive charge and electrons a single negative charge, while neutrons have no charge. Atoms have no overall electric charge because the number of protons equals the number of electrons.

Question 2: An isotope of radium is 22688Ra. Which statement about the nucleus of this isotope is correct?

1. The number of protons is 88.
2. The number of neutrons is 88.
3. The number of protons is 226.
4. The number of neutrons is 226.
5. The number of electrons is 226.

Question 2 answer: 1.
The isotope of radium has a mass number of 226, which means that the number of protons plus neutrons in each atom totals 226. It has an atomic number of 88, which means that the number of protons in each atom is 88. Subtracting the atomic number from the atomic mass tells us that there are 138 neutrons.

The rest of the questions

Question 3: A fast-moving particle passes close to the nucleus of an atom but is not affected by it. What is the particle most likely to be?

1. a proton
2. an alpha particle
3. a negative ion
4. an electron
5. a neutron

Question 3 answer: 5.
If a fast-moving particle passes close to the positively charged nucleus of an atom but is not affected by it, this means that the particle is not charged so that there are no attractive or repulsive forces. It is a neutron.

Question 4: The nuclei of carbon-14 atoms decay by emitting beta radiation. Which one of these statements is correct?

1. The carbon-14 nuclei split in two.
2. The carbon-14 nuclei emit hydrogen atoms.
3. A smaller nucleus of carbon is produced.
4. The atomic mass number of the carbon nuclei increases.
5. The carbon-14 nuclei emit electrons.

Question 4 answer: 5.
In beta emission, a neutron within the nucleus is converted into a proton and an electron, and the electron is emitted. This means that the atomic mass number of the carbon-14 nuclei does not change as they emit beta radiation.

Question 5: A student suggests that background radiation can come from: 1. outer space 2. rocks in the ground 3. human beings themselves. Which of the suggestions is/are correct?

1. 1 and 2
2. 2 only
3. 1 and 3 only
4. 1 only
5. 1, 2 and 3

Question 5 answer: 5.
Background radiation can come from all three of these sources.

Question 6: The count rate of a radioactive source decreases from 1600 counts per minute to 400 counts per minute in 12 hours. What is the half-life of the source?

1. 1.5 hours
2. 3 hours
3. 4 hours
4. 12 hours
5. 6 hours

Question 6 answer: 5.
The activity of the source halves from 1600 cpm to 800 cpm and then from 800 cpm to 400 cpm. This takes 12 hours and covers two half-lives. The half-life of the source is therefore 6 hours.

Question 7: Iodine-131 is a radioactive material with a half-life of 8 days. A sealed box holds 16 mg of iodine-131. How much iodine-131 will be left after 24 days?

1. 2 mg
2. 4 mg
3. 8 mg
4. 12 mg
5. 16 mg

Question 7 answer: 1.
The mass of iodine-131 halves during each half-life. 24 days covers 3 half-lives, so the mass falls from 16 mg to 8 mg to 4 mg to 2 mg.