If the complexity of a molecule increases, what happens to its entropy?

When the complexity of a molecule increases, its entropy generally increases as well. Entropy is a measure of the number of possible microstates that a system can occupy, which is a reflection of the system's disorder. More complex molecules have more atoms and bonds, leading to a greater variety of ways the constituents can arrange themselves and distribute energy.

For example, as you add more atoms to a molecule or introduce new structural features (like branching in organic compounds), you create more vibrational, rotational, and translational modes that the molecule can occupy. This increase in the number of microstates contributes to a higher entropy.

In contrast, simpler molecules, with fewer atoms and less complexity, have fewer ways to arrange themselves and thus lower entropy. Consequently, the relationship between molecular complexity and entropy reflects the underlying principles of thermodynamics, where greater disorder correlates with higher entropy values.