A) all these are colourless
B) all these are coloured
C) only \[N{{i}^{2+}}\] is coloured and \[Z{{n}^{2+}}\]and \[C{{r}^{3+}}\] are colourless
D) only \[Z{{n}^{2+}}\] is colourless and \[N{{i}^{2+}}\] and \[C{{r}^{3+}}\] are coloured
Correct Answer: D
Solution :
The ions having unpaired electrons are coloured. \[Z{{n}^{2+}}=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{0}},3{{d}^{10}}\] \[C{{r}^{3+}}=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{0}},3{{d}^{3}}\] \[N{{i}^{2+}}=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{0}},3{{d}^{8}}\] \[\therefore \] Only \[Z{{n}^{2+}}\] is colourless as it has all electrons paired. \[\therefore \,C{{r}^{3+}}\] and \[N{{i}^{2+}}\] are coloured due to the presence of unpaired electrons.
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