Question

$ { CrO }_{ 5 } $  has structure as shown, the oxidation number of chromium in the compound is?

(A). +4

(B). +5

(C). +6

(D). +10

(E). 0

Answer 1

Hint: We need to find the oxidation number of chromium (Cr). So, just count the number of peroxide bonds and double bonded oxygen. Then calculate the oxidation number of chromium accordingly.

Complete step by step answer:

We can see that in the structure of  $ { CrO }_{ 5 } $  one Cr atom is attached to five oxygen atoms. It makes a double bond with one Oxygen atom and single bonds with the other four Oxygen atoms .

Oxygen being more electronegative attracts the shared electron towards itself. So, it is obvious that the oxidation number of Cr atoms must be positive.

Let us imagine the oxidation number of Cr be X.

Four oxygen atoms are connected to Cr through a single bond. Hence the oxidation state of the oxygen atom in this case is -1. The last oxygen atom is attached through a double bond. So, this has oxidation number -2

We know that the sum of all oxidation numbers turns up to be 0.

Therefore,  $ X\quad +\quad (-2)\quad +\quad 4(-1)\quad =\quad 0 $ 

Solving this we will get,  $ X\quad =\quad 6 $ 

Hence we can say oxidation number of  $ Cr\quad =\quad +6 $ 

Therefore, the correct option is (C).

Note: If we solve this question traditionally, without drawing the structure of CrO5 we will get the answer +10 , which is obviously wrong.

 $ X\quad +\quad 5(-2)\quad =\quad 0 $ 

 $ X\quad =\quad 10 $ 

So, always draw the structure while identifying the oxidation number.