What NEET Asks
- Questions typically focus on the general trends, reasons for irregularities, and exceptions in d-block atomic radii.
- Comparisons between elements from different transition series (e.g., 3d vs 4d vs 5d) are frequent.
- Expect questions on the impact of d-electron shielding and the crucial concept of lanthanoid contraction.
Key Points
- General Trend: Across a transition series, atomic radii initially decrease, then become nearly constant, and finally show a slight increase towards the end.
- Initial Decrease: Primarily due to increasing effective nuclear charge (Zeff) as new electrons add to the same d-subshell, pulling the electron cloud inward.
- Mid-Series Constancy: A delicate balance between increasing Zeff and the simultaneously increasing shielding effect of the accumulating d-electrons.
- End-Series Increase: Occurs due to enhanced inter-electronic repulsion between the completely filled or nearly filled d-orbitals, which starts to outweigh the increased Zeff.
- d-Electron Shielding: d-electrons provide relatively poor shielding compared to s and p electrons, contributing to a higher Zeff experienced by outer electrons.
- Lanthanoid Contraction: Causes 4d and 5d series elements (of the same group) to have very similar atomic radii due to the poor shielding by 4f electrons, which precede the 5d series.
Must-Know Formula / Reaction
- Atomic Radius is governed by the interplay of Effective Nuclear Charge (Zeff) and Shielding Effect (σ).
- Zeff = Z - σ
- Zeff: The net positive charge experienced by an electron.
- Z: Atomic number (number of protons).
- σ: The shielding constant, representing the reduction of Zeff by inner electrons.
Common Mistakes
- Students often assume a continuous decrease in atomic radii across a transition series, ignoring the mid-series constancy and end-series increase.
- Don't confuse the poor shielding of d-electrons with strong shielding; it's the poor shielding that contributes to the unique trends.
- Forgetting to apply the concept of Lanthanoid Contraction when comparing 4d and 5d series elements, leading to incorrect size order.
Rapid Revision
Atomic radii in d-block elements exhibit an irregular trend: decrease initially (Zeff), then become constant (Zeff vs. shielding balance), and finally show a slight increase (electron-electron repulsion). Remember that d-electrons shield poorly, and lanthanoid contraction makes 4d and 5d radii almost identical.