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Which one of the following ions has electronic configuration $[Ar]3d^6$? (At. nos. Mn = 25, Fe = 26, Co = 27, Ni = 28)
A
$Co^{3+}$
B
$Ni^{3+}$
C
$Mn^{3+}$
D
$Fe^{3+}$
Detailed Solution
When a transition metal forms a cation, the 4s electrons are removed before the 3d electrons.
Co (27): $[Ar]3d^74s^2$; $Co^{3+}$: remove two 4s and one 3d electron, giving $[Ar]3d^6$
Ni (28): $[Ar]3d^84s^2$; $Ni^{3+}$: $[Ar]3d^7$
Mn (25): $[Ar]3d^54s^2$; $Mn^{3+}$: $[Ar]3d^4$
Fe (26): $[Ar]3d^64s^2$; $Fe^{3+}$: $[Ar]3d^5$
Hence the ion with $[Ar]3d^6$ is $Co^{3+}$.
Co (27): $[Ar]3d^74s^2$; $Co^{3+}$: remove two 4s and one 3d electron, giving $[Ar]3d^6$
Ni (28): $[Ar]3d^84s^2$; $Ni^{3+}$: $[Ar]3d^7$
Mn (25): $[Ar]3d^54s^2$; $Mn^{3+}$: $[Ar]3d^4$
Fe (26): $[Ar]3d^64s^2$; $Fe^{3+}$: $[Ar]3d^5$
Hence the ion with $[Ar]3d^6$ is $Co^{3+}$.