Why do heavier isotopes tend to be less abundant?

Heavier isotopes tend to be less abundant because their formation requires more energy and specific conditions during nuclear reactions.

In more detail, isotopes are variants of a particular chemical element which differ in neutron number, and thus in atomic mass. The most common isotopes are usually those with the lowest atomic mass, or the 'lightest' isotopes. This is because they require less energy to form. During nuclear reactions, such as those that occur in stars, lighter elements are fused together to create heavier elements. This process, known as nuclear fusion, requires a significant amount of energy. The heavier the element, the more energy is needed for its formation.

Furthermore, the formation of heavier isotopes often requires very specific conditions. For example, many of the heavier elements on the periodic table were formed during supernovae, when a star explodes at the end of its life. This event provides the extreme conditions necessary for the formation of these heavier isotopes. However, such events are relatively rare in the universe, which contributes to the lower abundance of heavier isotopes.

In addition, heavier isotopes are often less stable than their lighter counterparts. They are more likely to undergo radioactive decay, a process in which they break down into lighter elements and isotopes. This further reduces their abundance in the universe.

Lastly, the process of stellar nucleosynthesis, which is responsible for creating most of the chemical elements in the universe, naturally favours the creation of lighter elements. This is due to the specific reactions involved in this process, which are more likely to produce elements with lower atomic masses. Therefore, heavier isotopes are less abundant because they are harder to create, require specific conditions to form, are less stable, and are less favoured in the processes that create elements.