What's the difference between a chiral and achiral molecule?

A chiral molecule is asymmetric and non-superimposable on its mirror image, while an achiral molecule is symmetric and superimposable on its mirror image.

In more detail, chirality is a property of a molecule that cannot be superimposed onto its mirror image. This means that if you were to place a mirror next to the molecule, the reflection would look different from the original molecule. This is due to the arrangement of atoms in the molecule. Chiral molecules often contain a carbon atom with four different groups attached to it, known as a chiral centre or stereocentre. These molecules are also optically active, meaning they can rotate plane-polarised light. This property is significant in many areas of chemistry and biochemistry, as the two forms (or enantiomers) of a chiral molecule can have different chemical behaviours.

On the other hand, achiral molecules are symmetric and can be superimposed onto their mirror images. This means that the molecule and its reflection in a mirror would look identical. Achiral molecules do not have a chiral centre and are optically inactive as they do not rotate plane-polarised light. Even if a molecule has one or more chiral centres, it can still be achiral if it has an internal plane of symmetry, also known as a mirror plane. This makes the molecule's overall structure symmetric.

In summary, the key difference between chiral and achiral molecules lies in their symmetry and the ability to be superimposed on their mirror images. This difference can significantly affect the molecule's chemical properties and interactions, particularly in biological systems where chirality can influence the effectiveness of drugs and the function of biological molecules.