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Q4.35 (i):
Compare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points: (i) electronic configurations

Solution :

1. Theoretical Foundation of d-Block Electronic Configurations

The transition elements belong to the $d$-block, characterized by the progressive filling of $(n-1)d$ orbitals. The general valence shell configuration is $(n-1)d^{1-10}ns^{1-2}$. The comparison between the first ($3d$), second ($4d$), and third ($5d$) transition series highlights anomalies driven by relativistic effects, shielding constants, and exchange energy stability.

2. Comparative Analysis of Valence Configurations

While members of the same group exhibit similar valence shell configurations, the principal quantum number $n$ increases down the group ($3d$, $4d$, and $5d$). The configuration of the $n$-th period corresponds to the $(n-1)d$ filling sequence.

Group 3d Series ($n=4$) 4d Series ($n=5$) 5d Series ($n=6$)
Group 6 Cr: $[Ar] 3d^5 4s^1$ Mo: $[Kr] 4d^5 5s^1$ W: $[Xe] 4f^{14} 5d^4 6s^2$
Group 10 Ni: $[Ar] 3d^8 4s^2$ Pd: $[Kr] 4d^{10} 5s^0$ Pt: $[Xe] 4f^{14} 5d^9 6s^1$

3. Step-by-Step Analysis of Configuration Trends

Step 1: The Principle of $n$-Level Shift
The $3d$ series involves filling the $3d$ subshell with $4s$ valence electrons. The $4d$ and $5d$ series follow the same Aufbau principle logic, but the $5d$ series is preceded by the filling of the $4f$ subshell ($[Xe] 4f^{14} 5d^{1-10} 6s^{1-2}$), which significantly impacts the chemical properties due to the lanthanoid contraction [Per the Slater's Rules regarding shielding efficacy].

Step 2: Analysis of Anomalous Configurations
Within vertical columns, the energy gap between $(n-1)d$ and $ns$ orbitals decreases as we move from $3d$ to $5d$.

  • In the $3d$ series, half-filled stability ($d^5$) and fully-filled stability ($d^{10}$) are common (e.g., $Cr$, $Cu$).
  • In the $4d$ and $5d$ series, these anomalies become more frequent and erratic. For example, $Pd$ ($4d^{10} 5s^0$) completely promotes $s$-electrons to the $d$-orbital to achieve maximum symmetry, a phenomenon less common in its lighter congeners.

4. Visual Representation of Orbital Energy Levels

3d Series 4d Series 5d Series n-1 n n+1 Energy Level

5. Justification of Differences

The electronic configurations of $4d$ and $5d$ elements exhibit less deviation from the expected Aufbau pattern than the $3d$ series in many cases, yet they show more complex hybridization potential. The $5d$ series, specifically, is characterized by the presence of the $4f$ inner-transition block, which provides poor shielding of the nuclear charge, leading to smaller atomic radii and higher ionization enthalpies compared to the $4d$ series [Per the Lanthanoid Contraction hypothesis].

Final Solution: The electronic configurations of the transition series follow the $(n-1)d^{1-10}ns^{1-2}$ pattern, but the $5d$ series is distinct due to the intervention of the $4f^{14}$ shell. Consequently, while vertical congeners share valence electron counts, the $4d$ and $5d$ series demonstrate a narrower energy gap between orbitals and more frequent promotion of $ns$ electrons to the $(n-1)d$ shell compared to the $3d$ series.


More Questions from Class 12 Chemistry The d- and f-Block Elements Exercises


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