1: What is valency ? Mention use of valency?

       Valency is the combining capacity of an element.  Valency can be used to find out how many atoms of an element will combine with the other element to form a chemical formula. For example, hydrogen has a valency +1 and chlorine has a valency –1, so one atom of hydrogen combines with one atom of chlorine to form hydrochloric acid (HCl). Oxygen has a valency –2, so one atom of oxygen combines with two atoms of hydrogen to form water molecule (H₂O). 

2: What are the rules of writing a chemical formula? or, How is a molecular formula of a compound written?

       In order to write formula of a compound comprised of cations and anions, the valencies of the ion must be known. Let us consider a compound composed of cation A with valency x+ and anion B with valency y–. Then the molecular formula can be written by following the rules given below: 

      (i) Write down the symbols of the cation and anion side by side.
           A B
       (ii) Write their valencies at the top corners as
           Ax+ By–
       (iii) The valencies or charges on the ions must balance after combining. So, interchange between the ions their valencies and these are placed on the lower side of each radical or used as subscripts.
           Ay Bx
        (iv) If a radical is multi-atomic, use a small bracket around it before writing the valency number to indicate the ratio.
         (v) Eliminate the common factor, if any, from the numbers used in subscripts. For example, calcium phosphate, a compound of calcium ion with valency 2+ and phosphate ion with valency 3– is written as Ca₃(PO₄)₂.

3: Give the chemical name, chemical formulae for the following compounds having common names?
Baking soda, Washing soda, Blue vitriol, Green vitriol, Gypsum, Oil of vitriol or White vitriol, Soda ash, Marble, Lime water?

                
4: Name the smallest particle that has all the properties of an element?

     Atom.

5: How many ions are present in one mole of ions ?

    6.023 x 1023