JEE Main 2020 — Chemistry Inorganic Chemistry

2020mcqeasy

The pair in which both the species have the same magnetic moment (spin only) is:

Official previous-year question

Held on 4 Sept 2020 · Verified 6 Jul 2026.

Options

A
${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$ and ${[\mathrm{Fe}{({H}_{2}O)}_{6}]}^{2+}$
B
${[\mathrm{Co}{(\mathrm{OH})}_{4}]}^{2–}$ and ${[\mathrm{Fe}{({\mathrm{NH}}_{3})}_{6}]}^{2+}$
C
${[\mathrm{Mn}{({H}_{2}O)}_{6}]}^{2+}$ and ${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$
D
${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$ and ${[{\mathrm{CoCl}}_{4}]}^{2–}$

	No. unpaired ${e}^{-}$
${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}\Rightarrow {\mathrm{Cr}}^{2+}\Rightarrow 3{d}^{4}\Rightarrow {{t}_{2g}}^{1,1,1},{{e}_{g}}^{1,0}$	$4$
${[\mathrm{Fe}{({H}_{2}O)}_{6}]}^{2+}\Rightarrow {\mathrm{Fe}}^{2+}\Rightarrow 3{d}^{6}\Rightarrow {{t}_{2g}}^{1,1,1},{{e}_{g}}^{1,1}$	$4$
${[\mathrm{Fe}{({\mathrm{NH}}_{3})}_{6}]}^{2+}\Rightarrow {\mathrm{Fe}}^{2+}\Rightarrow 3{d}^{6}\Rightarrow {{t}_{2g}}^{2,1,1},{{e}_{g}}^{1,1}$	$4$
${[\mathrm{Co}{(\mathrm{OH})}_{4}]}^{2-}\Rightarrow {\mathrm{Co}}^{2+}\Rightarrow 3{d}^{7}\Rightarrow {{e}_{g}}^{2,2},{{t}_{2}}^{1,1,1}$	$3$
${[{\mathrm{CoCI}}_{4}]}^{2-}\Rightarrow {\mathrm{Co}}^{2+}\Rightarrow 3{d}^{7}\Rightarrow {e}^{2,2},{{t}_{2}}^{1,1,1}$	$3$
${[\mathrm{Mn}{({H}_{2}O)}_{6}]}^{2+}\Rightarrow {\mathrm{MN}}^{2+}\Rightarrow 3{d}^{5}\Rightarrow {{t}_{2g}}^{1,1,1},{{e}_{g}}^{1,1}$	$5$
So ${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$ and ${[\mathrm{Fe}{({H}_{2}O)}_{6}]}^{2+}$	have same magnetic moment (spin only)

Work through every JEE Main Inorganic Chemistry PYQ, year by year.

Options

${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$ and ${[\mathrm{Fe}{({H}_{2}O)}_{6}]}^{2+}$

${[\mathrm{Co}{(\mathrm{OH})}_{4}]}^{2–}$ and ${[\mathrm{Fe}{({\mathrm{NH}}_{3})}_{6}]}^{2+}$

${[\mathrm{Mn}{({H}_{2}O)}_{6}]}^{2+}$ and ${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$

${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$ and ${[{\mathrm{CoCl}}_{4}]}^{2–}$

Solution

	No. unpaired ${e}^{-}$
${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}\Rightarrow {\mathrm{Cr}}^{2+}\Rightarrow 3{d}^{4}\Rightarrow {{t}_{2g}}^{1,1,1},{{e}_{g}}^{1,0}$	$4$
${[\mathrm{Fe}{({H}_{2}O)}_{6}]}^{2+}\Rightarrow {\mathrm{Fe}}^{2+}\Rightarrow 3{d}^{6}\Rightarrow {{t}_{2g}}^{1,1,1},{{e}_{g}}^{1,1}$	$4$
${[\mathrm{Fe}{({\mathrm{NH}}_{3})}_{6}]}^{2+}\Rightarrow {\mathrm{Fe}}^{2+}\Rightarrow 3{d}^{6}\Rightarrow {{t}_{2g}}^{2,1,1},{{e}_{g}}^{1,1}$	$4$
${[\mathrm{Co}{(\mathrm{OH})}_{4}]}^{2-}\Rightarrow {\mathrm{Co}}^{2+}\Rightarrow 3{d}^{7}\Rightarrow {{e}_{g}}^{2,2},{{t}_{2}}^{1,1,1}$	$3$
${[{\mathrm{CoCI}}_{4}]}^{2-}\Rightarrow {\mathrm{Co}}^{2+}\Rightarrow 3{d}^{7}\Rightarrow {e}^{2,2},{{t}_{2}}^{1,1,1}$	$3$
${[\mathrm{Mn}{({H}_{2}O)}_{6}]}^{2+}\Rightarrow {\mathrm{MN}}^{2+}\Rightarrow 3{d}^{5}\Rightarrow {{t}_{2g}}^{1,1,1},{{e}_{g}}^{1,1}$	$5$
So ${[\mathrm{Cr}{({H}_{2}O)}_{6}]}^{2+}$ and ${[\mathrm{Fe}{({H}_{2}O)}_{6}]}^{2+}$	have same magnetic moment (spin only)