Final answer:
Functional groups are clusters of atoms within molecules that significantly increase the polarity, molecular mass, and reactivity, but not the chain length of the molecule. They introduce heteroatoms, increasing electronegativity which affects electron distribution.
Step-by-step explanation:
A functional group is a specific group of atoms within an organic molecule that imparts characteristic chemical reactivity to the molecule. These groups introduce heteroatoms such as oxygen or nitrogen and can significantly increase the molecule's reactivity. The introduction of such groups to hydrocarbon chains does indeed increase the polarity, molecular mass, and reactivity of the molecule, but it doesn't necessarily increase the chain length as that is dependent on the carbon skeleton of the molecule itself.
Polarity is increased because the heteroatoms usually included in functional groups (like oxygen, nitrogen, sulfur, phosphorus, or halogens) are more electronegative than carbon and hydrogen. Molecular mass also increases with the addition of these heteroatoms. Reactivity is a major feature that is affected by the introduction of functional groups due to changes in electron distribution within the molecule. However, changing a functional group does not directly increase the length of a carbon chain; rather, it alters the overall chemical behavior of the molecule.