| Date | May 2015 | Marks available | 2 | Reference code | 15M.2.HL.TZ1.9 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Explain and Identify | Question number | 9 | Adapted from | N/A |
Question
Part 2 Radioactivity
Radium-224 \(\left( {{}_{88}^{224}{\rm{RA}}} \right)\) is a radioactive nuclide that decays to form radon-220. Radon-220 is itself radioactive and undergoes a further decay. The table shows the series of radioactive nuclides that are formed as the decays proceed. The series ends with a stable isotope of lead.
For the final thallium nuclide, identify the
(i) nucleon number.
(ii) proton number.
Radon-220 is a radioactive gas. It is released by rocks such as granite. In some parts of the world, houses are built from materials containing granite. Explain why it is unlikely that radon-220 will build up in sufficient quantity to be harmful in these houses.
(i) Calculate, in hour−1, the decay constant of lead-212.
(ii) In a pure sample of lead-212 at one instant, 8.0 × 10−3 kg of the lead-212 is present. Calculate the mass of lead-212 that remains after a period of 35 hours.
(iii) A sample of pure radium begins to decay by the series shown in the table. At one instant, a mass of 8.0 × 10−3 kg of lead-212 is present in the sample. Suggest why, after 35 hours, there will be a greater mass of lead-212 present in the sample than the value you calculated in (h)(ii).
Markscheme
(i) 208;
(ii) 81;
because the half-life is (only) 55 s;
radon is produced slowly but decays quickly (so cannot build up);
(i) \(\left( {\lambda = \frac{{{\rm{In2}}}}{{{{\rm{T}}_{\frac{1}{2}}}}} = \frac{{0.693}}{{10.6}} = } \right)6.5 \times {10^{ - 2}}{\rm{ hou}}{{\rm{r}}^{ - 1}}\)
(ii) use of λ from (h)(i);
correct substitution into N = N0e−λt ;
8.0 to 8.3 × 10–4 kg;
(iii) the rate of decay/activity of polonium/radium;
is greater than the rate of decay/activity of lead;
