# Formula

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In mathematics and in the sciences, a formula (formulas or formulae in plural form) is a concise way of expressing information symbolically (as in a mathematical or chemical formula), or a general relationship between quantities. One of many famous formulae is Albert Einstein's E=mc² (see special relativity).

When you attempt to write a chemical formula, it is important to know whether the substance in question actually exists. For example, one can easily write the formula of carbon nitrate, but no chemist has ever prepared this compound. Here are basic rules for writing chemical formulae:

1. Represent the symbols of the components, placing the positive part first, and then the negative part.

2. Indicate the respective oxidation numbers above and to the right of each symbol. (Enclose radicals in parentheses for the time being.)

3. For each symbol, write a subscript number equal to the oxidation number of the other element or radical. This is the same as the mechanical crisscross method. Since the positive oxidation number shows the number of electrons that may be lost or shared and the negative oxidation number shows the number of electrons that may be gained or shared, you must have just as many electrons lost (or partially lost in sharing) as are gained (or partially gained in sharing).

4. Now rewrite the formulas, omitting the subscript 1, the parentheses of the radicals that have the subscript 1, and the plus and minus numbers.

5. As a general rule, the subscript numbers in the final formula are reduced to their lowest terms. There are, however, certain exception, such as hydrogen peroxide(H2O2, and acetylene C2H2). For these exceptions, you must have more specific information about the compound.

The only way to become proficient at writing formulae is to memorize the oxidations numbers of common elements (or learn to use the periodic chart group numbers) and practice writing formulae.