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Question

Which of the following arrangements does not represent the correct order of property stated against it ?

A
V2+<Cr2+<Mn2+<Fe2+: paramagnetic behaviour
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B
Ni2+<Co2+<Fe2+<Mn2+: Ionic size
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C
Co3+<Fe3+<Cr3+<Se3+: Stability in aqueous solution
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D
Sc<Ti<Cr<Mn:number of oxidation states
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Solution

The correct option is A V2+<Cr2+<Mn2+<Fe2+: paramagnetic behaviour
Answer: a
(a)
V2+=3 unpaired electrons
Cr2+=4 unpaired electrons
Mn2+=5 unpaired electrons
Fe2+=4 unpaired electrons

Hence, the order of paramagnetic behaviour should be
V2+<Cr2+=Fe2+<Mn2+
(a) is incorrect.

(b) Ionic size decreases from left to right along a period till Ni2+ for 3d series.
So, the given order for ionic size is correct.

(c) (As per data from NCERT)
Standard reduction potentials
Co3+/Co2+=1.97 V;
Fe3+/Fe2+=0.77 V;
Cr3+/Cr2+=0.41 V

Sc3+ is highly stable , attains a noble gas configuration. So it does not show +2 oxidation state.
Higher the SRP, lesser is the stability of +3 in aqueous medium since the tendency is to reduce.
So the given order is correct.

(d) The Oxidation states increases as we go from group 3 to group 7 in the same period.
Sc has +3 oxidation states.
Ti has +2,+3,+4 oxidation states.
Cr has +2,+3,+4,+5,+6 oxidation states.
Mn has +2,+3,+4,+5,+6,+7 oxidation states.
The given order is correct.

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