The Sequence Rule R and

The sequence rule (also known as the Cahn-lngold-Prelog (CIP) system) is the universal system of describing absolute configurations. It provides a method of arranging atoms or groups in an order of precedence and is used to assign stereochemical descriptors, R-, S-, also E-, Z-, and others. 

In the following diagrams the order of priority of the groups is a b c d. Hence, the molecule is viewed from opposite group d. 

The rules as they apply to compounds with centres of chirality may be summarised as follows  

Atoms of higher atomic number take precedence over those of lower atomic number. Thus, C1 S 0 N C H. Lone pairs are assigned the lowest possible priority. 

Isotopes of higher atomic weight take precedence over those of lower atomic weight. Thus, 

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