A Level Physics Practice Exam

What is true about stable isotopes with regard to their nuclei?

1. They disintegrate over time

2. They are held together by the weak nuclear force

3. Their nuclei do not disintegrate and are held together by a force

4. They consist solely of protons

The correct answer is: Their nuclei do not disintegrate and are held together by a force

Stable isotopes are characterized by their nuclei that do not undergo disintegration over time, which means they do not emit radiation or change into other elements or isotopes. This stability arises from the balance between the attractive nuclear forces, primarily the strong nuclear force, that hold the protons and neutrons in the nucleus together, and the repulsive electromagnetic force that acts between protons due to their positive charge. In stable nuclei, the number of neutrons provides the necessary force to help offset the repulsion between protons, allowing the nucleus to remain intact and stable. This stability is a key feature of stable isotopes, as opposed to unstable isotopes, which have an imbalance in this ratio and thus undergo radioactive decay. The other options refer to concepts that do not apply to stable isotopes. Disintegration refers to unstable isotopes that undergo radioactive decay. The weak nuclear force is not primarily responsible for holding the nucleus together; rather, it plays a minor role in certain types of particle interactions, such as beta decay. Finally, while stable isotopes consist of protons and neutrons, stating that they are made solely of protons is incorrect as stable nuclei require the presence of neutrons to maintain balance and stability.