Answer :

the atomic number itself won't say you a thing about valency of an element.atomic number say you about the charge on the nucleus  and also the number of electron in neutral atom.
but when the elements are arranged in periodic table you can determine the number of valance electrons from the position of element in the table  the Roman numeral at the top of each element will give the number of valance electron
Shravani83
Calculation of Valency:
1.To determine the valency of an element,we need to remember the periodic table.
2. Calculate the valency of an element using the total number of electrons. 
[Remember:All the elements in group VIII have eight electrons in their outer shells, and thus have a valency of zero]
3. Calculate the valency of multi-element molecules using the same procedure.

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