what is criss cross method of writing formula
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:
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While writing the chemical formula, certain rules have to be kept in mind. These rules are given below:
The valencies or charges on the ions must be balanced.
In case of a compound consisting of a metal and a non-metal, the symbol of the metal is written first. For example, in calcium chloride (CaCl2) and zinc sulphide (ZnS), calcium and zinc are metals, hence they are written first, whereas chlorine and sulphur are non-metals.
(iii) In case of compounds consisting of polyatomic ions, the polyatomic ions are enclosed in a bracket before writing the number to indicate the ratio. For example, in aluminium sulphate [Al2 (SO4)3], the polyatomic sulphate ion 
is enclosed in a bracket before writing the subscript 3.
(a) Formula of calcium oxide
Symbol
Ca
O
Charge
2+
2−
Thus, the chemical formula of calcium oxide is CaO.
(b)Formula of aluminium sulphate
Symbol
Al
SO4
Charge
3+
2−
Thus, the formula of aluminium sulphate is Al2(SO4)3. Brackets are used when we have two or more of the same ions in a formula. Here, the bracket with a subscript 3 indicates that three sulphate groups are joined to two aluminium atoms.