Revision: D and F Block Elements >> D-block Elements Chemistry HSC Science (General) 12th Standard Board Exam Maharashtra State Board

Elements having partially filled d-orbitals in ground state or in excited state are known as transition elements.

• Located between s-block and p-block elements
• Occupy Groups 3 to 12
• Present in 4 periods (4th to 7th)

Series of d-block:

Atomic and Ionic Radii:

• Atomic and ionic radii of d-block elements are smaller than s-block but larger than p-block elements.
• Within a 3d series, atomic radii decrease for the first five elements (Sc to Mn), then remain almost constant for the next five (Fe to Zn). This is because the increase in ENC (effective nuclear charge) first causes shrinkage, but additional d-electrons increase shielding and counterbalance further shrinkage.
• The 4d and 5d series elements have larger atomic and ionic radii than 3d series elements (due to more electron shells). However, 4d and 5d elements have nearly the same size — due to lanthanoid contraction.

Atomic Volume and Density:

• Atomic volume decreases along a period (as atomic size decreases).
• Density increases along the period.

Melting and Boiling Points:

• All transition elements have high melting points (typically above 900°C) in their solid state.
• Zn, Cd, Hg have abnormally low melting points because their completely filled d-orbitals prevent strong covalent metallic bonding.
• As unpaired electrons increase, metallic bonding strengthens → higher melting point. Tungsten (W) has the highest melting point of all metals.
• Mn and Tc have abnormally low melting points.

Enthalpies of Atomisation:

• Due to strong interatomic attraction, transition metals have high enthalpies of atomisation.
• Greater the number of valence electrons → stronger metallic bonding → higher enthalpy of atomisation.
• Members of 4d and 5d series have greater enthalpy of atomisation than 3d series.

Ionisation Energies:

• IE values of d-block elements lie between those of s-block and p-block elements.
• IE first increases up to Mn, then becomes irregular or constant due to the irregular trend of atomic size in 3d series.
• IE of Zn, Cd, and Hg are abnormally high due to the greater stability of completely filled d-subshells.
• The first two IE values of Ni are lower than Pt → Ni(II) compounds are more thermodynamically stable than Pt(II).

IE₁ order (important anomalies):

1. Hg > Cd > Zn
2. Au > Cu > Ag
3. Pt > Pd > Ni

Oxidation States:

All transition elements except the first and last of each series show a number (variable) of oxidation states.

• Mn shows the maximum number of oxidation states in the first series (7 states) — because it has 5 unpaired 3d electrons + 2 s-electrons available.
• Higher oxidation states are more stable for heavier members of a group (e.g., Mo(VI) and W(VI) are more stable than Cr(VI)).
• Lower oxidation states are more stable for lighter (3d) members.

Standard Electrode Potential:

• No regular trend exists in E° (M²⁺/M) values because IE and sublimation enthalpies show irregular variation.
• SRP tends to become more positive across a period (left to right) due to increasing IE and decreasing atomic size.
• Within a group, SRP becomes more negative going down.

• E° for Ni²⁺/Ni and Zn²⁺/Zn are more negative than expected. The high negative value of Ni²⁺/Ni stabilises Ni²⁺ ions. The high negative value for Zn²⁺/Zn is due to the stable, completely filled 3d¹⁰ configuration.
• Cr²⁺ is a strong reducing agent (acts as a reducing agent, gets oxidised to Cr³⁺; the d³ configuration = t₂g³ is very stable).
• Mn³⁺ (d⁴) is an oxidising agent — it gets reduced to Mn²⁺ (d⁵), which has an exactly half-filled d-orbital (extra stability).
• E°(Mn²⁺/Mn) is more negative than expected — due to extra stability of the half-filled 3d⁵ (Mn²⁺) ion.

Coloured Ions: Most of the transition metal compounds (ionic as well as covalent) are coloured both in the solid and in aqueous solution, in contrast to the compounds of s and p-block elements.

Magnetic Properties: In the case of transition metals, as they contain unpaired electrons in (n – 1)d orbitals, most of the transition metal ions and their compounds are paramagnetic.

Magnetic moment is calculated by spin only formula viz.

\[\mu=\sqrt{n\left(n+2\right)}\mathrm{~B.M.}\]

where n = number of unpaired electrons