THE STABILITY OF ISOTOPES
4 marks · NormalThe table below gives information about three isotopes of lithium.
| Isotope | Protons | Neutrons |
|---|---|---|
| Lithium-6 | 3 | 3 |
| Lithium-7 | 3 | 4 |
| Lithium-8 | 3 | 5 |
State, for all isotopes of lithium, what determines that they are all lithium atoms.
Explain what effect increasing the number of neutrons has on the stability of the lithium nucleus. Use examples from the table.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) The number of protons (atomic number) determines the element; all lithium atoms have 3 protons.
(b) Increasing the number of neutrons can make the nucleus less stable. Lithium-6 and lithium-7 are stable, but lithium-8 (with 5 neutrons) is unstable and decays. Too many neutrons causes instability and leads to radioactive decay.
Common mistakes: Mixing up neutron number and atomic number; Incorrectly saying more neutrons always increase stability
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Marking points
B1 (a) · 1 mark · States that the number of protons/atomic number determines the element/lithium.
M1 (b) · 1 mark · Refers to stability being affected by neutron number.
A1 (b) · 1 mark · Uses table data (e.g. lithium-6 and lithium-7 stable; lithium-8 unstable).
A2 (b) · 1 mark · States that too many neutrons leads to instability/decay.
Worked solution
a) The number of protons in an atom determines the element. Every lithium atom has 3 protons; this is its atomic number.
b) As the number of neutrons increases, the stability of the nucleus can decrease. For example, lithium-6 (3 neutrons) and lithium-7 (4 neutrons) are stable, but lithium-8 (5 neutrons) is unstable and will decay. This happens because too many neutrons upset the balance of nuclear forces, making the nucleus unstable, so it becomes radioactive and decays.