THE STABILITY OF ISOTOPES
3 marks · NormalExplain why heavy elements like uranium have more neutrons than protons in their most stable isotope.
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In heavy elements, there are more protons in the nucleus, and the repulsive electrical force between protons increases. Extra neutrons are needed to provide more of the strong nuclear force, which helps to counteract the increasing repulsion and keep the nucleus stable. Therefore, the most stable isotopes of heavy elements have more neutrons than protons.
Also accept: Heavier elements have more nuclear repulsion so they need extra neutrons
Common mistakes: Thinking the number of protons and neutrons is always equal; Not referring to balancing forces
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Marking points
B1 · 1 mark · States that extra neutrons are needed to stabilize heavy nuclei.
M1 · 1 mark · Mentions increased repulsion between many protons.
M2 · 1 mark · Explains that extra neutrons provide additional strong nuclear force.
Worked solution
Heavy elements like uranium have many protons in their nuclei, which strongly repel each other. Neutrons add to the strong nuclear force, which binds the nucleus, but do not add to the repulsive force, so more neutrons are needed in heavy elements to maintain nuclear stability.