Question

The aqueous solution containing which one of the following ions will be colourless? (Atomic number of $Sc = 21, Fe = 26, Ti = 22, Mn = 25$)

• A. $Sc^{3+}$
• B. $Fe^{2+}$
• C. $Ti^{3+}$
• D. $Mn^{2+}$

Answer 1

Answer: (A)

(a) ${ }_{21} Sc ^{3+}=1 s^{2}, 2 s^{2} 2 p^{6}, 3 s^{2} 3 p^{6}$
It is colourless due to absence of unpaired electrons in $d$ -subshell.
(b) $26 Fe ^{2+}=1 s^{2}, 2 s^{2} 2 p^{6}, 3 s^{2} 3 p^{6} 3 d^{6}$
It is colourless due to presence of four unpaired electrons in $d$ -subshell.
(c) ${ }_{22} Ti ^{3+}=1 s^{2}, 2 s^{2} 2 p^{6}, 3 s^{2} 3 p^{6} 3 d^{1}$
It is coloured due to presence of one unpaired electron in $d$ -subshell.
(d) ${ }_{25} Mn ^{2+}=1 s^{2}, 2 s^{2} 2 p^{6}, 3 s^{2} 3 p^{6} 3 d^{5}$
It is coloured due to $5$ unpaired electrons in d-subshell.