Question

(A) Write the electronic configuration of the element with atomic number 24.
(B) Find the number of unpaired electrons in $Co^{2+$ ion.}
(C) Why is the range of oxidation states greater in the actinoid series compared to the lanthanoid series?

Hint:

Remember the exceptions in electronic configurations for transition metals: Cr (24) and Cu (29) both promote an s-electron to the d-subshell to achieve half-filled ($d^5$) or fully-filled ($d^{10}$) stability.

Answer 1

(A) Element with Z = 24 (Chromium):
Expected Configuration: $[Ar] 3d^4 4s^2$
Actual Configuration: $[Ar] 3d^5 4s^1$
(Reason: Half-filled d-orbitals provide extra stability). (B) Unpaired electrons in $Co^{2+$:}
Atomic number of Cobalt ($Co$) = 27. Configuration of $Co$: $[Ar] 3d^7 4s^2$. Configuration of $Co^{2+}$: $[Ar] 3d^7$. Orbital representation of $3d^7$: \framebox{$\uparrow\downarrow$} \framebox{$\uparrow\downarrow$} \framebox{$\uparrow$} \framebox{$\uparrow$} \framebox{$\uparrow$} Number of unpaired electrons = 3. (C) Oxidation States in Actinoids vs Lanthanoids:
The actinoids show a greater range of oxidation states than lanthanoids because the energy gap between the 5f, 6d, and 7s subshells is very small. This allows electrons from all these subshells to participate in bond formation. In contrast, the 4f orbitals in lanthanoids are buried deep inside and participate less in bonding, limiting their oxidation states primarily to +3.