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In chemistry (especially organic chemistry), a molecule is chiral (and said to have chirality) if its overall structure and overall three-dimensional configuration is always chiral in accordance with the geometric definition (it can't be mapped to its mirror images) regardless of how the molecule is conformed. Conformations are temporary positions a molecule can assume as a result of bond rotation, bending, or stretching as long as a bond is not broken to change the molecule.
An atom is often said to be chiral if the atom is a chiral center in a molecule. An atom is a chiral center when the molecule it is in, regardless of the molecule's conformation, can't be made to be identical to (or super-imposable on) its mirror image by rotations and translations alone if the chiral atom's center must be super-imposable on its own mirror image position. A molecule can have multiple chiral centers without being chiral overall. Also, it is possible, but not very common, for a molecule to have a local area that is not an atom that acts effectively as a chiral center anyway due to an unusual shape the molecule may have. It is also possible for a molecule's overall shape to be chiral without any specific chiral center points in the molecule. An example is given by 1,3-dichloro-allen, characterized by 2 double bonds on the same carbon, outlining two perpendicular planes. For this molecule it is possible to write two enantiomers lacks a chiral center.
Molecular isomers that are enantiomorphs of each other are called enantiomers. Because such molecules typically show optical activity, they are also often called optical isomers, The study of chiral molecules, enantiomers (optical isomers), and molecules with chiral atoms is part of the science of stereochemistry.
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