Why do we say that water is h2o and not oh2

There are two common methods for writing chemical formulas.

Explanation:

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 CH4 and H2O.

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 H2O, NaCl, and H2SO4 (not ClH or H2O4S).

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.