Question

Different (variable) oxidation state is shown by transition elements. It is due to the fact:
A. (n-1)d electrons may be excited to ns orbital
B. ns electrons may be excited to (n-1)d orbitals
C. (n-1)d electrons may get involved along with ns electrons in bonding
D. None of the above.

Answer 1

Hint: There are certain elements in the periodic table, which have partially filled d subshells. These elements can form cations with incompletely filled d orbitals. They are called transition elements.

Complete step by step answer:
Elements which are in group 3 to 12, including lanthanides and actinides come under the group transition elements. These elements come under the d-block and their general electronic configuration is ${\text{(n - 1)}}{{\text{d}}^{1 - 10}}{\text{n}}{{\text{s}}^{1 - 2}}$. These elements may have electrons in both ns and (n-1)d orbitals. The energy difference between these two orbitals is very small. So, while forming bonds, these elements first use ns electrons and then if required, (n-1)d electrons can also be used.
Hence, variable oxidation states are shown by transition elements due to the fact that (n-1)d electrons may get involved along with ns electrons in bonding.

So, option C is correct.

Additional information:
Each transition element shows a minimum and a maximum oxidation state. The minimum oxidation state corresponds to the total number of electrons in s subshell whereas, maximum oxidation state corresponds to the total number of electrons available in both ns and (n-1)d subshell. The number of oxidation states shown by transition elements are –

Elements	Outer electronic configuration	Oxidation states
Sc	${\text{3}}{{\text{d}}^1}{\text{ 4}}{{\text{s}}^2}$	+2, +3
Ti	${\text{3}}{{\text{d}}^2}{\text{ 4}}{{\text{s}}^2}$	+2, +3, +4
V	${\text{3}}{{\text{d}}^3}{\text{ 4}}{{\text{s}}^2}$	+2, +3, +4, +5, +6
Cr	${\text{3}}{{\text{d}}^5}{\text{ 4}}{{\text{s}}^1}$	+2, +3, +4, +5, +6,
Mn	${\text{3}}{{\text{d}}^5}{\text{ 4}}{{\text{s}}^2}$	+2, +3, +4, +5, +6, +7
Fe	${\text{3}}{{\text{d}}^6}{\text{ 4}}{{\text{s}}^2}$	+2, +3, +4, +5, +6
Co	${\text{3}}{{\text{d}}^7}{\text{ 4}}{{\text{s}}^2}$	+2, +3, +4
Ni	${\text{3}}{{\text{d}}^1}{\text{ 4}}{{\text{s}}^2}$	+2, +3, +4
Cu	${\text{3}}{{\text{d}}^{10}}{\text{ 4}}{{\text{s}}^1}$	+1, +2
Zn	${\text{3}}{{\text{d}}^{10}}{\text{ 4}}{{\text{s}}^2}$	+2

Note: Cr, Cu, Ag, Au and Hg are the elements which show minimum oxidation state of 1. Stability of oxidation states increases in the order 3d ˂ 4d ˂ 5d. 3d series elements are most stable in +2; 4d series in +2 and +4 and 5d series in +4.