What is the significance of the proton-neutron ratio in isotopes?

The proton-neutron ratio determines the stability of an isotope.

In more detail, the stability of an isotope is heavily influenced by the ratio of protons to neutrons in its nucleus. Protons are positively charged particles, and neutrons are neutral particles. Both are found in the nucleus of an atom. The forces that hold the nucleus together, known as the strong nuclear force, must overcome the repulsive forces between the positively charged protons. Neutrons help to stabilise the nucleus by adding attractive forces without adding repulsive forces, as they are neutral.

For lighter elements, a roughly equal number of protons and neutrons tends to result in a stable nucleus. For example, carbon-12, which has 6 protons and 6 neutrons, is stable. However, as elements get heavier, more neutrons are needed to counteract the increasing repulsive forces between the larger number of protons. This is why heavier elements like uranium-238, which has 92 protons and 146 neutrons, have a higher neutron-to-proton ratio.

If the proton-neutron ratio is too far from the ideal range, the isotope becomes unstable and may undergo radioactive decay to reach a more stable state. This decay can involve the emission of alpha particles, beta particles, or gamma rays, depending on the type of instability. For instance, carbon-14, which has 6 protons and 8 neutrons, is unstable and undergoes beta decay to become nitrogen-14.

Understanding the proton-neutron ratio is crucial in fields like nuclear physics and chemistry, as it helps scientists predict the behaviour of different isotopes and their potential uses or hazards. For GCSE Physics students, grasping this concept is fundamental to understanding why certain elements are radioactive and how nuclear reactions occur.