THE STABILITY OF ISOTOPES
4 marks · HardThe isotope oxygen-19 has 8 protons and 11 neutrons. Oxygen-16 is a stable isotope. Explain, using scientific reasoning, why oxygen-19 is less stable than oxygen-16.
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Must show: oxygen-19 has a higher neutron to proton ratio than stable oxygen isotopes, extra neutrons upset the balance of forces and make nucleus unstable, too many neutrons can cause beta decay, oxygen-16 has optimal ratio for stability
Common mistakes: stating mass alone makes isotopes unstable; confusing neutron number with proton number
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Marking points
B1 · 1 mark · States oxygen-19 has more neutrons / higher neutron-to-proton ratio than oxygen-16
B2 · 1 mark · Explains that additional neutrons make the nucleus unstable
B3 · 1 mark · Links instability to imbalance in the interplay between nuclear and electrostatic forces
B4 · 1 mark · Mentions that unstable nuclei may decay (e.g. by beta decay) to reach a more stable ratio
Worked solution
Oxygen-16 is stable because it has an equal number of protons and neutrons, giving it an optimal balance between the attractive nuclear force and the repulsive electrostatic force between protons. Oxygen-19 has 8 protons and 11 neutrons, so its neutron-to-proton ratio is higher than in stable oxygen. Too many neutrons disturb the force balance in the nucleus, so the nuclear forces are no longer sufficiently greater than the electrostatic repulsion to keep all nucleons bound. This makes oxygen-19 unstable, and it may undergo beta decay to convert a neutron to a proton, reducing its neutron number and moving towards a stable ratio.