meritnation experts , i'm unable to understand the writing of a chemical formula for a compound . its a common question in our exams and that where i lose marks .

i studied the study material but it all went over my head. can u explain it to me again with examples (like calcium oxide etc ).

pls try answering fast. even other fellow students can answer . 

Chemical formula of a compound is the symbolic representation of its atomic constituents. In other words, a chemical formula represents the composition of a molecule in terms of the symbols of the elements present in that molecule.

To write the chemical formula of a compound, one should have prior knowledge of two things:

1. The symbols of the constituent elements, and
2. The combining capacity of each atom constituting the compound which is termed as valency

 The number of atoms of other elements with which one atom of an element combines is decided by the valency of that element.

Example:

In calcium oxide

1. Calcium has +2 valency while oxygen has -2 valency, these valencies got cancelled by eachother, so formula is CaO.

2. 

Now this is clear to you!