Concept 01 / 06The d and f Block Elements
Transition Elements: Position, Electronic Configuration and General Trends
Position of d-block elements, definition of transition elements, general electronic configuration and exceptions (Cr, Cu), why Zn, Cd and Hg are not typical transition elements, and trends in metallic character, melting points, enthalpy of atomisation, atomic and ionic radii (effect of lanthanoid contraction) and ionisation enthalpies.
The d-block occupies groups to , in which the orbitals of the penultimate shell are filled. There are four series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg) and 6d (Ac, Rf to Cn).
Transition elements. Elements that have an incompletely filled d subshell in their ground state or in any common oxidation state. Zn, Cd and Hg have configurations in the ground state and in their common state, so they are not regarded as typical transition elements (though they are d-block elements). Copper () is a transition element because is .
Electronic configuration. General: . For the 3d series: Sc , Ti , V , Cr , Mn , Fe , Co , Ni , Cu , Zn . The exceptions (Cr, Cu) arise because half-filled and completely filled d subshells have extra stability (exchange energy) and and are close in energy. On ionisation, electrons are lost first.
Physical properties. Nearly all are hard, lustrous metals with high tensile strength, malleability and good conductivity. Melting and boiling points and enthalpies of atomisation are high (see figure), because both and electrons take part in metallic bonding; they rise to a maximum around the middle of each series () and then fall. Mn and Tc have anomalously low values. Zn has the lowest (all d electrons paired). Heavier series have higher enthalpies of atomisation (stronger metal-metal bonding), which is why the 4d and 5d metals form more metal-metal bonded compounds.
Atomic and ionic radii. Radii decrease across a series (increasing nuclear charge with poor shielding by d electrons) but only slightly in the middle, and increase a little at the end (electron repulsion as d fills). Down a group, radii increase from 3d to 4d, but the 4d and 5d elements have almost identical radii (Zr , Hf ; Nb and Ta; Mo and W). This is due to the lanthanoid contraction: the poorly shielding electrons filled before the 5d series cause the expected increase to be cancelled.
Density increases across the series as radius falls and mass rises (Sc to Cu ).
Ionisation enthalpies. Increase gradually across the series (increasing nuclear charge, partly offset by shielding), less steeply than for main group elements. Irregularities reflect the stability of and : the second IE of Cr and Cu is very high ( and are broken), and the third IE of Mn is very high ( is ). Zn has a high first IE (filled ). The 5d elements have higher IEs than 4d because of the lanthanoid contraction.
Key Takeaways:
- Transition element: incomplete d subshell in the atom or a common ion; Zn, Cd, Hg are not typical.
- Cr and Cu are exceptions.
- High melting points with a maximum near ; Mn anomalously low, Zn lowest.
- 4d and 5d radii nearly equal (Zr ≈ Hf) due to lanthanoid contraction.
- Pitfall: 4s electrons are filled before 3d, but they are also removed first on ionisation.
Brain-Teaser 1: Which of these are transition elements by definition: Sc, Zn, Cu, Ag, Cd?
Solution: Sc ( is , but the atom is : yes), Cu ( is : yes), Ag ( in is : yes). Zn and Cd are in both the atom and the ion: not transition elements.
Brain-Teaser 2: Why are the atomic radii of Zr and Hf almost the same, and what is the practical consequence?
Solution: Between Zr (4d) and Hf (5d), the lanthanoids intervene; their electrons shield poorly, so the effective nuclear charge rises and the radius contracts by about as much as the extra shell would increase it. Because Zr and Hf have nearly identical size and chemistry, they occur together and are extremely hard to separate.
Brain-Teaser 3: Why is the enthalpy of atomisation of zinc the lowest in the 3d series?
Solution: In Zn () all the d electrons are paired and held tightly, so they do not participate in metallic bonding; only the electrons contribute. Weaker metallic bonding gives the lowest enthalpy of atomisation and melting point ().
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