The d-block elements are divided into the first transition series (Sc through Cu elements), the second transition series (Y through Ag elements), and the third transition series (La elements and Hf through Au elements). Actinium, Ac, is the first member of the fourth transition series, including Rf through Rg as well.
The f-block elements are the Ce by Lu elements that make up the lanthanide series (or lanthanoid series) and the Th by Lr elements that make up the actinide series (or actinoid series). Lanthanum is considered as lanthanide element, although its configuration of electrons makes it the first member of the third transition series. Likewise, actinium's behavior means that it is part of the actinide series, although its configuration of electron makes it the first member of the fourth transition series.
Properties and Trends in Transition Metals
The elements of the Periodic Table's second and third rows show gradual property changes across the table as expected from left to right. Due to the addition of protons in the nucleus, electrons in the outer shells of the atoms of these elements have little shielding effects leading to an increase in effective nuclear charge. The effects on atomic properties are therefore: smaller atomic radius, increased energy for first ionization, increased electronegativity, and more non-metallic character. This trend continues until calcium (Z=20) is reached. At this point, there's an abrupt break. In their physical and chemical properties, the next ten elements called the first transition series are remarkably similar. In terms of their relatively small difference in effective nuclear charge over the series, this general similarity in properties has been explained. This happens as each additional electron enters the penultimate 3d shell that provides an effective shield between the nucleus and the shell of the outer 4s.
The physical and chemical properties of transition elements differ from the main group elements (s-block). Properties of transition elements are discussed below:
The density and hardness of transition elements are high due to the small size of their atoms and the strong metallic bonding. With the exception of mercury, which at room temperature is a liquid, all other elements are solid metals with all the characteristics of a metal. The elements of transition are much denser than the elements of the s-block and show a gradual increase in density from scandium to copper. The small and irregular decrease in metallic radii coupled with the relative increase in atomic mass can explain this trend in density.
Transition metals usually have very high melting and boiling points due to the presence of strong metallic bonds in transition metals that occur as a result of the delocalization of electrons facilitated by both d and s electrons being available.
Zn, Cd and Hg metals have lower melting and boiling points because there are no unpaired electrons available because they have completely filled d orbitals. These metals do not undergo covalent bonding due to unavailability of unpaired electrons. The remaining transition metals have both metallic and covalent bonding. Metals have the highest melting point towards the middle of each transition series due to strong metal bonding.
The transition elements' first ionization energy is higher than s-block elements but lower than p-block elements. While ionization energy increases gradually in a particular transition series as we move from left to right, this increase is not appreciable. The increase in ionization energy is due to an increase in the nuclear charge. The effect of an increase in the nuclear charge is partially balanced by an increase in screening effect. The increase in ionization energy during the d-block elements period is therefore very small.
In transition elements, the addition of the extra electron in the (n-1) d level occurs when moving along the period. This electron creates a screening effect and shields from the nucleus pull at the outer ns electrons. The effect of nuclear charge (effective nuclear charge) on external ns electrons is somewhat less than the actual nuclear charge due to this shielding effect of d electrons. Thus, the effects of the increasing nuclear charge and the shielding effect created by (n-1)d orbital expansion are opposed to each other. Because of these counter-effects, the potential for ionization increases slowly during the first transition series
First Ionization Energy: For the first four 3d series elements (Sc, Ti, V and Cr), the first ionization energy is almost identical, i.e. it differs only slightly from each other. First ionization energy values are also very close to each other for Fe, Co, Ni and Cu. Due to the extra-stability of the fully filled 3d10 level, the value of the first ionization energy for Zn is significantly high.
Second ionization energy: With the increase of atomic number, the second ionization appears to increase more or less regularly. The value of Cr and Cu's second ionizing energy is higher than their neighbors' value. This is because the Cr+ and Cu+ ion electronic configurations have extra stable levels of 3d5 and 3d10.
In going from II B (Zn-group elements) to IIIA sub-group, there is a sudden drop in the values of ionization potential. This is because in the case of IIIA group elements the electron to be removed is from an incompletely filled 4p-orbital, whereas in the case of IIB group elements the electron to be removed is from a fully filled 4s-orbital which requires extra energy.
In contrast to those of s and p block elements, many compounds of transition elements are coloured. The d-orbitals of transition elements are not degenerated in compound state due to the surrounding groups (ligands), but divided into two groups of different energy.
Therefore, electrons can be promoted from one group to another. This corresponds to a relatively small difference in energy, so light is absorbed in the visible region. Some transition metal compounds are white, such as ZnSO4 and TiO2. The electrons within the d-level cannot be promoted in these compounds.
Transition metals show a wide variety of chemical behaviors. Some transition metals are strong reducing agents, as can be seen from their reduction potential (Table P1), while others have very low reactivity. All lanthanides form stable 3 + aqueous cations. For such oxidations, the driving force is similar to alkaline earth metals e.g. Be or Mg, which form Be2+ and Mg2+. On other hand, materials such as platinum and gold have significantly higher potential for reduction. Their oxidation resistance makes them useful materials for circuit making and jewelry making.
Alloys are homogeneous two or more metal solid solutions obtained by melting the components and then cooling the melt. These are formed by metals whose atomic radii differ by no more than 15%, so that one metal's atoms can easily take up the positions in the other's crystal lattice. As transition metals have similar atomic radii, they are very ready to form alloys.
Elements of transition show variable oxidation status unlike elements of s and p block. The oxidation indicates changes in one unit, e.g. Fe2 + & Fe+3, Cu+1 & Cu+2.
Scandium can have an oxidation number (+ II) when both s electrons are used for bonding and (+ III) when there are two s and one d electrons. Similarly, all elements show variable oxidation states depending on the number of electrons in their s and d sub-shells available for bonding.
Transition metals display variable oxidation status with unique properties. These properties stems from the fact that the energy levels of 3d, 4d, and 5d orbitals are very close, respectively, to those of 4s, 5s, and 6s orbitals, and therefore electrons from both ns and (n-1)d orbitals can be used for formation and transition metal bonds.
Minimum oxidation state: All transition elements except Cr, Cu, Ag, Au and Hg with a minimum +1 oxidation state have a minimum + 2 oxidation state.
Maximum oxidation state: The maximum oxidation status of each of the elements in groups III B to VII B is equal to its group number. Cr in group VIB, for example, shows a maximum oxidation state in Cr2O7 2 –ion of + 6.
Most of the VIII group elements show a maximum state of oxidation equal to + 6. Ru and Os, however, have a maximum state of oxidation equal to +8 which is the highest state of oxidation shown by any element.