<P> A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using a single line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and plus (+) and minus (−) signs . </P> <P> Compounds may be described using formulas in various formats . For compounds that exist as molecules, the formula for the molecular unit is shown . For polymeric materials, such as minerals and many metal oxides, the empirical formula is normally given, e.g. NaCl for table salt . </P> <P> The elements in a chemical formula are normally listed in a specific order, called the Hill system . In this system, the carbon atoms (if there are any) are usually listed first, any hydrogen atoms are listed next, and all other elements follow in alphabetical order . If the formula contains no carbon, then all of the elements, including hydrogen, are listed alphabetically . There are, however, several important exceptions to the normal rules . For ionic compounds, the positive ion is almost always listed first and the negative ion is listed second . For oxides, oxygen is usually listed last . </P> <P> In general, organic acids follow the normal rules with C and H coming first in the formula . For example, the formula for trifluoroacetic acid is usually written as C HF O. More descriptive formulas can convey structural information, such as writing the formula for trifluoroacetic acid as CF CO H. On the other hand, the chemical formulas for most inorganic acids and bases are exceptions to the normal rules . They are written according to the rules for ionic compounds (positive first, negative second), but they also follow rules that emphasize their Arrhenius definitions . To be specific, the formula for most inorganic acids begins with hydrogen and the formula for most bases ends with the hydroxide ion (OH). Formulas for inorganic compounds do not often convey structural information, as illustrated by the common use of the formula H SO for a molecule (sulfuric acid) that contains no H-S bonds . A more descriptive presentation would be O S (OH), but it is almost never written this way . </P>

The name of the compound formed by combining carbon atoms with oxygen atoms
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