ChemistryMediumClass 12

Transition Metal Trends — IE, atomic radius, melting point

d & f-block Elements

JEE Qs

Hard

min

Focus on understanding the underlying reasons (electronic configuration, Zeff, shielding, lanthanoid contraction, metallic bonding) for the trends and their exceptions, rather than just memorizing the patterns.

✅ Key Points for JEE

1Atomic Radii: Generally decrease across a period due to increasing effective nuclear charge (Zeff), with a slight increase towards the end due to increased d-d electron repulsion. Down a group, 4d series elements have slightly larger radii than 3d, but 5d series elements have nearly identical radii to their corresponding 4d elements due to Lanthanoid Contraction.
2Ionization Enthalpy (IE): Generally increases across a period, but exceptions exist due to stable d⁵ and d¹⁰ configurations (e.g., higher IE₂ for Cr and Cu). Down a group, IE values generally increase from 3d to 4d, and then significantly increase from 4d to 5d, primarily due to Lanthanoid Contraction and relativistic effects leading to a stronger attraction of valence electrons to the nucleus.
3Melting Point: Generally high for transition metals due to strong metallic bonding involving a large number of unpaired d-electrons. Melting points tend to peak around the middle of the series (Group 6, e.g., Cr, Mo, W) and decrease at the ends (Group 12, e.g., Zn, Cd, Hg) where d-orbitals are fully filled and fewer unpaired electrons are available for bonding. Mn and Tc show anomalously low melting points due to complex crystal structures.
4Density: Increases across a period due to decreasing atomic volume and increasing atomic mass. Densities are significantly higher for 5d elements compared to 4d and 3d elements due to Lanthanoid Contraction causing smaller atomic volumes while atomic masses are much higher.

⚠️ Common Mistakes

✕Ignoring the effect of Lanthanoid Contraction on the properties of 4d and 5d series elements, especially for atomic radii and ionization enthalpies.
✕Forgetting specific exceptions to the general trends, such as the unusual IE values for Cr/Cu or the low melting points of Mn/Tc/Zn, and failing to provide the electronic configuration reasoning.
✕Not linking melting point directly to the strength of metallic bonding and the number of unpaired d-electrons available for delocalization.
✕Assuming transition metals will follow the same simple periodic trends as s-block or p-block elements without considering the role of d-electrons and their shielding/repulsion effects.

NCERT Chapters

Class 11 Chemistry Ch 3: Classification of Elements and Periodicity in Properties
Class 12 Chemistry Ch 8: d- and f-Block Elements