Question

what will be the oxidation number of Cl in NOClO4 ?

Answer 1

NOClO4 is an ionic compound made up of polyatomic ions NO+ (nitric oxide) and ClO4- (perchlorate). 

Rules for working out oxidation number that apply here are: 
-the sum of oxidation numbers must equal the charge of polyatomic numbers 
-assign negative oxidation numbers to the most electronegative species 

Since ClO4 has a 1- charge, and we know each Oxygen's oxidation state is usually 2-, four oxygens amount to 8-. As the final charge of the perchlorate ion is 1-, that means Cl must have a positive charge of +7 to balance the negatives (ie 7-8 = -1). 

It may seem strange that Chlorine, a halogen has a positive charge, but remember charge depends on other species present. As oxygen is more electronegative than Chlorine, chlorine is forced to assume the positive charge and become the electron donator.
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Answer 2

Answer: The oxidation state of chlorine in the given compound is +7.

Explanation:

Oxidation number is defined as the number which is assigned to the element when it gains or loose electrons.

If the element gains electron, it will attain a negative oxidation state and if the element looses electrons, it will attain a positive oxidation state.

We are given a chemical compound having chemical formula of $NOClO_4$. This compound is made by the combination of $NO^+\text{ and }ClO_4^-$ ions.

The oxidation state of nitrogen in the ion is:

$x-2=+1\\x=+3$

We take the oxidation state of chlorine atom be 'x'.

Oxidation state of nitrogen atom = +3

Oxidation state of oxygen atom = -2

Evaluating the oxidation state of chlorine atom:

$+3+(-2)+x+4(-2)=0\\\\x=+7$

Hence, the oxidation state of chlorine in the given compound is +7.