O-LevelPhysicsNuclear Physics5054/22 · Oct/Nov 2019 · Q1016 Marks

All the isotopes of the gas radon are radioactive.

(a)State one similarity and one difference between the nuclei of two different isotopes of radon.[2]
(b)(i)The isotope radon-222 decays by alpha-particle emission to an isotope of polonium (Po). The proton number (atomic number) of polonium is 84. Determine the number of neutrons in an atom of the polonium isotope.[2]
(b)(ii)Determine the number of protons in an atom of radon-222.[2]
(b)(iii)Describe how a neutral atom of helium (⁴₂He) differs from an alpha-particle.[1]
(c)(i)In an experiment to collect a small quantity of helium, a sample of radon-222 is enclosed in an inner glass tube which has a very thin wall. [Figure 10.1] shows that this tube is placed inside a container that is initially evacuated. Both the container and the inner glass tube are sealed. As the radon-222 decays, alpha-particles pass through the thin wall of the inner glass tube. [Figure 10.2] shows how the total number of alpha-particles produced by the radioactive decay of the radon-222 changes as time passes. Use Fig. 10.2 to determine the number of alpha-particles produced in 7.6 days.[1]
(c)(ii)Initially, there are 2.8 × 10¹⁰ atoms of radon-222 in the inner glass tube. As each radon- 222 atom decays, it produces an alpha-particle. Calculate the number of radon-222 atoms that remain after 7.6 days.[1]
(c)(iii)Using the number of radon-222 atoms present initially and the number present after 7.6 days, calculate the half-life of radon-222.[3]
(d)The alpha-particles become helium atoms which are collected in the vacuum shown in Fig. 10.1. Explain, in terms of the properties of alpha-particles, why the wall of the inner glass tube must be extremely thin.[1]
(e)In some parts of the world, radon-222 accumulates in the air in buildings and is breathed in by people. Explain why the presence of an alpha-emitter in the lungs is particularly hazardous.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[B1] similarity: (same) number of protons
[B1] difference: (different) number of neutrons
[C1] (nucleon number =) 222-4 or 218 or 218-84
[A1] 134
[C1] (proton number =) 84 + 2
[A1] 86
[B1] it / helium atom has two electrons (in orbit around the nucleus)
[B1] 2.1 x 10¹⁰ (alpha-particles)
[B1] calculated value of: 2.8 x 10¹⁰ - candidate's 10(c)(i) (atoms remain) (expected value: 7.0 × 10⁹)
[C1] 7.0 x 10⁹ / 2.8 x 10¹⁰ or 1/4
[C1] 2 (half-lives) or 7.6 / 2
[A1] 3.8 days
[B1] alpha-particles are (very) weakly penetrating
[B1] strongly ionising
[B1] (ionisation causes cell) mutations / cancers