be applied across olefinic bonds (E- and Z- notation). Stereoisomers An important branch of stereochemistry is the study of chiral molecules, which are molecules that lack a plane of symmetry and are, therefore, not superimposable on their mirror images. The term chiral stems from the Greek word "cheir," meaning handedness and describes objects that have a "left-handed" and "right-handed" form. Molecules are considered to be chiral if they contain an asymmetric carbon atom, which is attached to four different substituents that form a tetrahedron. The idea of chirality is essential for explaining the concept of stereoisomerism. Compounds that have the same molecular formula, but differ in the spatial arrangement of their atoms are stereoisomers. Based on arrangement, these compounds can be categorized as either enantiomers or diastereomers. Enantiomers are pairs of stereoisomers that are non superimposable mirror images of each other. Comparably, diastereomers are stereoisomers which are superimposable on each other and are not mirror images. Stereoisomers that do not involve chirality are geometrical isomers, also known as cis-trans isomers, which exist as a result of restricted rotation around a double bond within a molecule. When two of the same atoms are attached to the same side of the molecule, a cis isomer is present. Conversely, when two of the same atoms are attached to opposing sides of the molecule, a trans isomer is present. Another type of stereoisomerism is conformational stereoisomerism. These isomers exist as a result of rotation around the central carbon-carbon bond within a molecule and are constantly being interconverted into its different isomers at room temperature. The possible conformational isomers are gauche (60°), anti (180°), and eclipsed (0°). Representation of Stereochemical Structures Wedge and dash diagrams are used to represent 3-dimensional molecules on paper and are often used to depict the stereochemistry of chiral molecules. Dashed wedges are used to show bonds that project behind the plane of the paper and dark and shaded wedges are used to show bonds that project out of the plan of the paper. The ordinary lines are used to show bonds that are in the plane of the paper. Fischer projections are a simplified way to represent 3-dimensional stereochemical molecules in 2-dimensional layout. All of the bonds are drawn as ordinary lines that intersect at 90°. The top and bottom lines represent the front and back of the molecule, respectively. One side line represents a dashed wedge and the other