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Answer 1 · 2016-11-29T01:20:05

There are two common methods for writing chemical formulas.

Method 1

Many chemists use the Hill system for writing chemical formulas,

There are two rules:

If the molecule contains carbon, the elements are listed in the order #"C"#, then #"H"#, then all the other elements in alphabetical order.
If there is no carbon, all the elements are listed alphabetically.

This gives us formulas like #"CH"_4# and #"H"_2"O"#.

This system is unambiguous and is used in most chemical databases and printed indexes to sort lists of compounds.

Method 2

Another common system puts the more positive elements in compound first.

This gives us formulas like #"H"_2"O"#, #"NaCl"#, and #"H"_2"SO"_4# (not #"ClH"# or #"H"_2"O"_4"S"#).

In practice, most chemists use Method 1 for organic compounds and Method 2 for inorganic compounds.

Nevertheless, it is permissible to depart from either method if you want to make a specific point.