Question

The oxidation number of Cr is +6 in __________
A.) $FeC{r_2}{O_4}$
B.) $KCr{O_3}Cl$
C.) $Cr{O_5}$
D.) ${\left[ {Cr{{\left( {OH} \right)}_4}} \right]^ - }$

Answer 1

Hint: We already know that oxidation number refers to the total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom. We have to consider the general oxidation number to calculate the oxidation number for Cr in each case.

Complete step by step answer:
To find the answer to the question given here, we must find the oxidation number of chromium in each of the coordination complexes given here. For that we need to follow certain rules:
The hydrogen atom (H) is always considered to exhibit an oxidation state of +1. However, when bonding with an element with less electronegativity than it, it is known to exhibit an oxidation number of -1.
Oxygen generally has an oxidation of -2 in most of its compounds. But, in the case of peroxides, the oxidation number of the oxygen is considered -1
For neutral compounds, the sum of all the oxidation numbers of the constituent atoms sums up to zero.
When polyatomic ions are considered, the sum of all the oxidation numbers of the atoms that form the ions equals to the net charge on the polyatomic ion.
Keeping this in mind, the oxidation number of Cr is considered to be x,
For $FeC{r_2}{O_4} = 2 + 2x + 4 \times ( - 2) = 0$
Or, $2x - 6 = 0$
Or, $x = + 3$

For $KCr{O_3}Cl = 1 + x + 3( - 2) - 1 = 0$
Or, $x - 6 = 0$
Or, $x = + 6$

Since, It has a butterfly structure, it has one oxygen with oxidation number -2 while the four others have peroxy linkage with oxidation number -1 each.
$Cr{O_5} = x - 2 + 4( - 1) = 0$
Or, $x - 6 = 0$
Or, $x = + 6$

${\left[ {Cr{{\left( {OH} \right)}_4}} \right]^ - }$ is an anionic compound, hence we will take the total sum of oxidation number as -1.

${\left[ {Cr{{\left( {OH} \right)}_4}} \right]^ - } = x + 4( - 2) + 4( + 1) = - 1$
Or, $x - 8 + 4 = - 1$
Or, $x - 4 = - 1$
Or, $x = + 3$

As we can see from the calculation, in and, the oxidation number of Cr is +6
Hence, the correct answer is Option (B) and Option (C).

Note: We must remember that oxidation numbers also play an important role in the systematic nomenclature of chemical compounds. This becomes exceedingly relevant in case of coordination complexes.