<P> The alkene called but - 2 - ene has two isomers, which the chemical formula CH CH = CHCH does not identify . The relative position of the two methyl groups must be indicated by additional notation denoting whether the methyl groups are on the same side of the double bond (cis or Z) or on the opposite sides from each other (trans or E). Such extra symbols violate the rules for chemical formulas, and begin to enter the territory of more complex naming systems . </P> <P> As noted above, in order to represent the full structural formulas of many complex organic and inorganic compounds, chemical nomenclature may be needed which goes well beyond the available resources used above in simple condensed formulas . See IUPAC nomenclature of organic chemistry and IUPAC nomenclature of inorganic chemistry 2005 for examples . In addition, linear naming systems such as International Chemical Identifier (InChI) allow a computer to construct a structural formula, and simplified molecular - input line - entry system (SMILES) allows a more human - readable ASCII input . However, all these nomenclature systems go beyond the standards of chemical formulas, and technically are chemical naming systems, not formula systems . </P> <P> For polymers in condensed chemical formulas, parentheses are placed around the repeating unit . For example, a hydrocarbon molecule that is described as CH (CH) CH, is a molecule with fifty repeating units . If the number of repeating units is unknown or variable, the letter n may be used to indicate this formula: CH (CH) CH . </P> <P> For ions, the charge on a particular atom may be denoted with a right - hand superscript . For example, Na, or Cu . The total charge on a charged molecule or a polyatomic ion may also be shown in this way . For example: H O or SO . </P>

When are parentheses needed in a chemical formula
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