Question 21M.2.HL.TZ2.4
| Date | May 2021 | Marks available | [Maximum mark: 10] | Reference code | 21M.2.HL.TZ2.4 |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Calculate, Draw, Explain, Write down | Question number | 4 | Adapted from | N/A |
During electron capture, an atomic electron is captured by a proton in the nucleus. The stable nuclide thallium-205 () can be formed when an unstable lead (Pb) nuclide captures an electron.
Write down the equation to represent this decay.
[2]
✓
✓
The unstable lead nuclide has a half-life of 15 × 106 years. A sample initially contains 2.0 μmol of the lead nuclide. Calculate the number of thallium nuclei being formed each second 30 × 106 years later.
[3]
calculates ✓
calculates nuclei remaining ✓
activity ✓
Accept conversion to seconds at any stage.
Award [3] marks for a bald correct answer.
Allow ECF from MP1 and MP2
Allow use of decay equation.
The neutron number N and the proton number Z are not equal for the nuclide . Explain, with reference to the forces acting within the nucleus, the reason for this.
[2]
Reference to proton repulsion OR nucleon attraction ✓
strong force is short range OR electrostatic/electromagnetic force is long range ✓
more neutrons «than protons» needed «to hold nucleus together» ✓
Thallium-205 () can also form from successive alpha (α) and beta-minus (β−) decays of an unstable nuclide. The decays follow the sequence α β− β− α. The diagram shows the position of on a chart of neutron number against proton number.
Draw four arrows to show the sequence of changes to N and Z that occur as the forms from the unstable nuclide.
[3]
any α change correct ✓
any β change correct ✓
diagram fully correct ✓
Award [2] max for a correct diagram without arrows drawn.
For MP1 accept a (−2, −2 ) line with direction indicated, drawn at any position in the graph.
For MP2 accept a (1, −1) line with direction indicated, drawn at any position in the graph.
Award [1] max for a correct diagram with all arrows in the opposite direction.