Question

In which of the following pairs, the oxidation states of sulfur and chromium are the same?
A) ${ SO }_{ 3 }^{ 2- }{ ,CrO }_{ 4 }^{ 2- }$
B) ${ SO }_{ 3 }{ ,CrO }_{ 4 }^{ 2- }$
C) ${ SO }_{ 2 }{ ,CrO }_{ 4 }^{ 2- }$
D) ${ SO }_{ 2 } { ,Cr }_{ 2 }{ O }_{ 7 }^{ 2- }$

Answer 1

Hint: The oxidation state of an atom is defined as the charge which it possesses in ionic form or appears to acquire when all the other atoms from the molecule are assumed to be removed as ions by counting the shared electrons with the more electronegative atom.

Complete step-by-step answer:
Transition elements show variable state oxidation in their compounds. The reason for the variable oxidation state is that there is a very small energy difference between ${ (n-1)d }$ and d-orbitals. As a result, electrons of ${ (n-1)d }$ orbitals as well as ns-orbitals take part in bond formation. Variation in the oxidation state is related to their electronic configuration.

The oxidation state of oxygen = ${ -2 }$. Therefore,
A) ${ SO }_{ 3 }^{ 2- }$
The oxidation state of sulfur = ${ x+3(-2) = -2 }$
Here, x = ${ +4 }$
${ CrO }_{ 4 }^{ 2- }$ - The oxidation state of chromium = ${ x+ 4(-2) = -2 }$
Here, x = ${ +6 }$

B) ${ SO }_{ 3 }$
The oxidation state of sulfur = ${ x+3 (-2) = 0 }$
Here, x = ${ +6 }$
${ CrO }_{ 4 }^{ 2- }$ - The oxidation state of chromium = ${ x+ 4(-2) = -2 }$
Here, x = ${ +6 }$

C) ${ SO }_{ 2 }$
 The oxidation state of sulfur = ${ x+2(-2) = 0 }$
Here, x = ${ +4 }$
${ CrO }_{ 4 }^{ 2- }$
The oxidation state of chromium = ${ x+ 4(-2) = -2 }$
Here, x = ${ +6 }$

D. ${ SO }_{ 2 }$
The oxidation state of sulfur = ${ x+2(-2) = 0 }$
Here, x = ${ +4 }$
${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }$
The oxidation state of chromium = ${ 2(x)+ 7(-2) } = { -2 }$
Here, x = ${ +6 }$
The oxidation states of sulfur and chromium are the same in ${ SO }_{ 3 }{ ,Cr }_{ 2 }{ O }_{ 7 }^{ 2- } i.e, { +6 }$

Hence, the correct option is B.

Note: The possibility to make a mistake is that you may choose option D. The oxidation state of sulfur dioxide is ${ +4 }$ and ${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }$ is ${ +6 }$, so the oxidation states of chromium and sulfur are not the same.