Formula

Formula :

The symbolic representation of one molecule of a compound representing the number of atoms of various elements present in it is called formula of the           compound.
Criss - cross Method:
Step I: Write the symbol of positive ion or radical to the left side and for the negative ion or radical to the right side.
Step II: Put the valency number of each radical or ion on its top right. Divide the valency numbers by highest common factor, if any, to get simple ratio. Now ignore (+) and (-) symbols. Interchange the valency numbers of radicals or ions as once superscript to others subscript and vice versa.
Step III: If the radicals receives a number more than one, enclose it within brackets. Donot enclose ions in brackets
Example 1 :    Step I: Ca²⁺, Cl^-       Step II:     Step III: CaCl₂.
Example 2:     Step I: Mg²⁺, OH^-     Step II:     Step III: Mg(OH)₂.

Information from the formula of a compound:    
When a numeral is written on the left hand side before the formula, it represents number of molecules of the compound. 
Example:  2NaCl, 4ZnO etc.
When the numeral is written on the right bottom side of the symbol, it represents the number of atoms in one molecule of a compound . 
 Example: Al₂(SO₄)₃. 
In this 2 atoms of aluminium , 3 atoms of sulphur and 12 atoms of oxygen are present..It is able to calculate its molecular weight by looking at a molecular formula.