# Question #15598

Nov 29, 2016

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 $\text{C}$, then $\text{H}$, then all the other elements in alphabetical order.
• If there is no carbon, all the elements are listed alphabetically.

This gives us formulas like ${\text{CH}}_{4}$ and $\text{H"_2"O}$.

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 $\text{H"_2"O}$, $\text{NaCl}$, and ${\text{H"_2"SO}}_{4}$ (not $\text{ClH}$ or $\text{H"_2"O"_4"S}$).

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.