In the first transition series ($3d$ elements), all elements from $\text{Ti}$ to $\text{Zn}$ have negative $E^\circ(\text{M}^{2+}/\text{M})$ values, except Copper. For copper, $E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}$. This unique positive value arises because the high enthalpy of atomization and high second ionization enthalpy of copper are not sufficiently balanced by its hydration enthalpy.

Calculation steps:

• Element with atomic number $Z=25$ is Manganese ($\text{Mn}$), ground state configuration: $[\text{Ar}]3d^5 4s^2$.
• The divalent ion is $\text{Mn}^{2+}$: Two electrons are lost from the $4s$ orbital, giving configuration $[\text{Ar}]3d^5$.
• The $3d^5$ subshell contains $n = 5$ unpaired electrons.
• Spin-only magnetic moment $\mu$ is calculated as:
  $$\mu = \sqrt{n(n+2)} = \sqrt{5(5+2)} = \sqrt{35} \approx 5.92\text{ BM}$$

Color in transition metal ions is caused by $d-d$ electronic transitions, which require partially filled $d$-orbitals ($d^1$ to $d^9$):

• $\text{Sc}^{3+}$ ($Z=21$): Electronic configuration is $[\text{Ar}]3d^0$. Since the $d$-subshell is completely empty ($d^0$), no $d-d$ transition can take place, making $\text{Sc}^{3+}$ colourless.
• $\text{V}^{4+}$: $[\text{Ar}]3d^1$ (blue colour).
• $\text{Mn}^{2+}$: $[\text{Ar}]3d^5$ (pale pink colour).
• $\text{Co}^{2+}$: $[\text{Ar}]3d^7$ (pink colour).