question_answer

The aqueous solution containing which one of the following ions will be colourless?  [AIPMT (S) 2005]

(Atomic no. : Sc = 21, Fe = 26, Ti = 22, Mn = 25)

A) \[S{{c}^{3+}}\]

B) \[F{{e}^{2+}}\]

C)      \[T{{i}^{3+}}\]

D) \[M{{n}^{2+}}\]

Correct Answer: A

Solution :

[a] \[_{21}Sc=1{{s}^{2}},2{{s}^{2}}\,2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}\,3{{d}^{1}},\,4{{s}^{2}}\]

So,       \[S{{c}^{3+}}=1{{s}^{2}},2{{s}^{2}}s{{p}^{6}},3{{s}^{2}}3{{p}^{6}}\]

(It is colourless due to absence of unpaired electrons in d-subshell)

\[_{26}Fe=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}3{{d}^{6}},4{{s}^{2}}\]

\[F{{e}^{2+}}=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},\,3{{s}^{2}}3{{p}^{6}}3{{d}^{6}}\]

(It is coloured due to presence of four unpaired electrons in d-subshell)

\[\begin{align}   & _{22}Ti=1{{s}^{2}},2{{s}^{2}}\,2{{p}^{6}},\,3{{s}^{2}}3{{p}^{6}}3{{d}^{2}},4{{s}^{2}} \\  & T{{i}^{3+}}=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}3{{d}^{1}} \\ \end{align}\]

(It is coloured due to presence of an unpaired electron in d-subshell)

\[\begin{align}   & _{25}Mn=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}3{{d}^{5}},4{{s}^{2}} \\  & M{{n}^{2+}}=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}3{{d}^{5}} \\ \end{align}\]

(It is coloured due to presence of unpaired electrons in d-subshell).