What is determined by the functional group of an organic molecule?

The functional group of an organic molecule plays a crucial role in determining its reactivity. Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They dictate how a molecule interacts with other substances, essentially defining how it can participate in chemical reactions.

For instance, hydroxyl groups (-OH) can make a molecule more polar and capable of forming hydrogen bonds, while carboxyl groups (-COOH) can act as acids, donating protons in solution. This reactivity influences the functions that the molecule can perform in biological systems or chemical processes.

While the other options—size, color, and molecular weight—are indeed properties associated with molecules, they are not primarily determined by the functional groups. Instead, size relates more to the number and arrangement of atoms, color can depend on the molecule's overall structure and electronic characteristics, and molecular weight is a function of the atom types and counts, rather than their specific functional groups. Thus, the functional group's direct influence on reactivity is what makes it the determining factor in this case.