Correct Option: (A) Both Assertion (A) and Reason (R) are correct and Reason (R) is the correct explanation of Assertion (A).
Step 1 (Definition of Transition Elements): [0.5 Mark]
According to the IUPAC definition, a transition element is defined as an element which has an incompletely filled $d$-orbital in its ground state or in any one of its common oxidation states. The general electronic configuration for these elements is represented as:
$$(n-1)d^{1-10} ns^{1-2}$$Step 2 (Electronic Configuration Analysis): [0.25 Mark]
Zinc ($Zn$), Cadmium ($Cd$), and Mercury ($Hg$) belong to Group 12. Their electronic configurations are:
- $Zn (Z=30): [Ar] 3d^{10} 4s^2$
- $Cd (Z=48): [Kr] 4d^{10} 5s^2$
- $Hg (Z=80): [Xe] 4f^{14} 5d^{10} 6s^2$
In their ground state and their common oxidation state ($+2$), the $(n-1)d$ orbitals are completely filled ($d^{10}$ configuration). Since they lack partially filled $d$-orbitals, they do not exhibit typical transition metal properties like variable oxidation states or colored ion formation.
Step 3 (Final Concluded Answer): [0.25 Mark]
Because the $(n-1)d$ subshell is completely filled in both the ground state and the common $+2$ oxidation state, these elements do not satisfy the criteria for transition elements. Thus, both the Assertion and the Reason are scientifically accurate, and the Reason correctly explains the Assertion.
💡 EL10 Concept Insight: Think of transition elements as "incomplete" puzzles; they need an empty spot in their $d$-orbital to be chemically active. Zinc, Cadmium, and Mercury have their $d$-orbitals completely "full," so they don't behave like the other transition metals!