Introduction
The d-block elements, including Zn, Cd and Hg, are termed transition metals or transition elements. Groups 3 to 12 of the periodic table are called transition elements. As per IUPAC, transition metal can be defined as an atom containing an incomplete d subshell or capable of forming cations containing an incomplete d-subshell. Thus, Zn, Cd and Hg are not considered transition metals as they don’t contain incomplete d-subshell (d10). These elements mostly form cations with a +2 oxidation state retaining their d10 configuration. So IUPAC considers elements of groups 3 to 11 as transition elements.
| Group No. | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
|---|---|---|---|---|---|---|---|---|---|---|
| Atomic No. | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 |
| Element | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
| Atomic No. | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 |
| Element | Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd |
| Atomic No. | 57 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 |
| Element | La | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg |
Elements of the fourth and fifth periods have empty f-subshells, whereas period 6 has completely filled f-subshells.
Electronic Configuration
The transition elements have (n-1)d0-10ns2 as their general electronic configuration. S-electrons are present in almost all elements. The orbital energies decrease as the charge on the nucleus increases from neon to calcium. Due to the poor penetrating ability of 3d orbitals, the 4s orbital penetrates deep into the argon core and has an energy level lower than the 3d level. Thus electrons enter the 3d orbitals only after the 4s orbitals get filled. A similar sort of behaviour is observed in the 4d series (Zr to Ag) and the 5d series (Hf to Au).
| Electronic Configuration | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu |
|---|---|---|---|---|---|---|---|---|---|
| M | 4s23d1 | 4s23d2 | 4s23d3 | 4s23d4 | 4s23d5 | 4s23d6 | 4s23d7 | 4s23d8 | 4s23d10 |
| M2+ | - | 3d2 | 3d3 | 3d4 | 3d5 | 3d6 | 3d7 | 3d8 | 3d9 |
| M3+ | [Ar] | 3d1 | 3d2 | 3d3 | 3d4 | 3d5 | 3d6 | 3d7 | 3d8 |
Common characteristics of transition elements
Many of the physical and chemical characteristics of d-block elements are similar. The physical properties of the d block involve a shining surface with high density and high melting and boiling points
| Metal | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
|---|---|---|---|---|---|---|---|---|---|---|
| Melting point (0C) | 1541 | 1668 | 1910 | 1857 | 1246 | 1538 | 1495 | 1455 | 1083 | 420 |
| Boiling point (0C) | 2836 | 3287 | 3380 | 2672 | 1962 | 2861 | 2870 | 2730 | 2567 | 907 |
| Density (g cm-3) | 2.99 | 4.54 | 6.11 | 7.19 | 7.33 | 7.87 | 8.90 | 8.90 | 8.92 | 7.13 |
All these properties indicate that these elements have strong cohesion forces giving rise to metallic bonds. The covalent bond binds only two atoms, while the metallic bond binds all atoms in metal together. They have good electrically and thermally conductive due to the presence of free electrons. They have high tensile strength and are malleable and ductile. They form alloys and have high enthalpies of vaporization. They exhibit paramagnetic properties due to the presence of spin-free electrons. They also exhibit efficient catalytic properties.
Chemical Characteristics
They are electropositive metals with variable vacancies. They combine with nonmetals to form several salts and complexes. They are reducing agents and form insoluble oxides, hydroxides and carbonates. Iron, cobalt, copper, molybdenum and zinc catalyze essential biochemical reactions.
