What is a chiral atom?

A chiral atom, most commonly a carbon atom, is characterized by having four different groups attached to it. This distinct arrangement leads to non-superimposable mirror images, known as enantiomers, which are a key concept in stereochemistry. The presence of four different substituents enables the formation of these unique spatial arrangements, which are significant in many areas including pharmaceuticals, where the chirality of a compound can influence its biological activity.

The other options do not fit the definition of chirality. For instance, an atom with two identical groups would not create unique mirror images because the presence of identical groups means there's no asymmetry. An atom bonded to a double bond does not meet the chirality requirement, as it typically cannot have four different groups even if it is connected to other atoms. Lastly, a symmetrical arrangement implies that the atom would not have the asymmetric properties needed for chirality, as symmetry would mean the mirror image is superimposable on the original. Thus, the defining feature of chirality is the presence of four different groups attached to the atom.