Atomic Radius of d-Block - NEET Notes, Formula & Common Mistakes
Atomic Radius d-Blockd and f Block ElementsNEET ChemistryNEET 2025d-electron shieldingLanthanoid ContractionTransition Elements
Atomic Radius of d-Block - NEET Notes, Formula & Common Mistakes
d and f Block Elements·2 min read·NEET 2026
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.
Frequently Asked Questions
Why do atomic radii vary irregularly across a d-block series?▾
The irregular trend is due to a delicate balance between increasing effective nuclear charge (Zeff), which tends to decrease radius, and the increasing shielding effect and inter-electronic repulsion of d-electrons, which tend to increase radius. This balance shifts across the series.
How do d-electrons influence the atomic size trend in transition metals?▾
d-electrons provide relatively poor shielding. Initially, increasing Zeff dominates, decreasing size. In the middle, the shielding effect of d-electrons partially counteracts Zeff. Towards the end, increased inter-electronic repulsion within the d-orbitals leads to a slight expansion.
What is the impact of Lanthanoid Contraction on the atomic radii of d-block elements?▾
Lanthanoid contraction, caused by the poor shielding of 4f electrons, leads to an unexpectedly small size for the 5d transition elements. Consequently, elements of the 4d and 5d series within the same group exhibit very similar atomic radii.
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