THE STABILITY OF ISOTOPES
3 marks · NormalA student investigates two isotopes of boron: boron-10 and boron-12. Boron-10 is stable, but boron-12 decays quickly after it is produced.
Explain, in terms of neutrons and protons, why boron-12 is less stable than boron-10.
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.
Show answer
Boron-10 has a balanced ratio of protons to neutrons, giving it stability. Boron-12 has more neutrons than protons, leading to an imbalance in nuclear forces. This imbalance makes boron-12's nucleus unstable, causing it to decay rapidly.
Also accept: States that boron-10 is stable because it has a suitable number of neutrons; boron-12 has too many neutrons, so it is unstable and decays
Common mistakes: Incorrectly stating that more neutrons is always better; Not mentioning stability/instability
Show mark scheme and worked solution
Marking points
M1 · 1 mark · States or implies that neutron-proton ratio affects stability.
A1 · 1 mark · Describes how boron-10 is stable because of a balanced ratio.
A2 · 1 mark · Explains that boron-12 has too many neutrons, making it unstable and prone to decay.
Worked solution
Boron-10 has 5 protons and 5 neutrons, so the ratio between protons and neutrons is balanced and the nucleus is stable.
Boron-12 has 5 protons but 7 neutrons. The extra neutrons cause an imbalance, disrupting the forces holding the nucleus together and making it unstable. This is why boron-12 decays quickly.