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Date May 2021 Marks available 2 Reference code 21M.2.HL.TZ2.4
Level Higher level Paper Paper 2 Time zone 2
Command term Write down Question number 4 Adapted from N/A

Question

During electron capture, an atomic electron is captured by a proton in the nucleus. The stable nuclide thallium-205 (Tl81205) can be formed when an unstable lead (Pb) nuclide captures an electron.

Write down the equation to represent this decay.

[2]
a.i.

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]
a.ii.

The neutron number N and the proton number Z are not equal for the nuclide Tl81205. Explain, with reference to the forces acting within the nucleus, the reason for this.

[2]
b.

Thallium-205 (Tl81205) 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 Tl81205 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 Tl81205 forms from the unstable nuclide.

[3]
c.

Markscheme

Pb82205 

e  -10AND   νe00 

 

a.i.

calculates λ=ln215×106 «= 4.62×10-8 year-1» 

calculates nuclei remaining 0.50×10-6×6.0×1023«=3.0×1017» 

activity =«λN=1.4×1010 nuclei per year»=440«nuclei per second» ✓


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.

a.ii.

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»  ✓

b.

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.

c.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
b.
[N/A]
c.

Syllabus sections

Core » Topic 7: Atomic, nuclear and particle physics » 7.1 – Discrete energy and radioactivity
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