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General chemical symbols

There are also symbols in chemical equations for groups of elements, for example in comparative formulas. These are often a single capital letter, and the letters are reserved and not used for names of specific elements. For example, " X " indicates a variable group (usually a halogen) in a class of compounds, while " R " is a radical, meaning a compound structure such as a hydrocarbon chain. The letter " Q " is reserved for "heat" in a chemical reaction. " Y " is also often used as a general chemical symbol, though it is also the symbol of yttrium. " Z " is also often used as a general variable group. " E " is used in organic chemistry to denote an electron-withdrawing group or an electrophile; similarly " Nu " denotes a nucleophile. " L " is used to represent a general ligand in inorganic and organometallic chemistry. " M " is also often used in place of a general metal.

At least two other, two-letter generic chemical symbols are also in informal use, " Ln " for any lanthanide and " An " for any actinide. " Rg " was formerly used for any rare gas element, but the group of rare gases has now been renamed noble gases and " Rg " now refers to roentgenium.