The atomic radii of the transition elements are intermediate between those of s- and p-block elements.
(a) The atomic radii of elements of a particular series decrease with increase in atomic number but this decrease in atomic radii becomes small after midway.
For example: For the elements of first transition series, the atomic radii decrease gradually from scandium to chromium but from chromium to copper, it remains practically constant.
The atomic radius decreases in a period in the beginning, because with increase in atomic number, the nuclear charge goes on increasing (by unity) progressively. However, we know that the electrons enter the penultimate (last but one) shell and the added d-electrons screen the outermost s-electrons. The shielding effect of d-electrons is small so that the net electrostatic attraction between the nuclear charge and the outermost electron increases. Consequently, the atomic radius decreases. As the number of d-electrons increases, the screening effect increases. This neutralises the effect of increased nuclear charge due to increase in atomic number, and consequently, atomic radius remains almost unchanged after chromium.
(b) At the end of each period, there is a slight increase in the atomic radii.
Explanation: Towards the end of the series, there are increased electron-electron repulsions between the added electrons in the same orbitals which exceed the attractive forces due to increased nuclear charge. Therefore, electron cloud expands and the size increases.
Up to chromium (3d5 4s1) electron-electron repulsions do not come into picture because there is no pairing of electrons in d-subshell. However, from iron onwards, the pairing of electrons (Fe: 3d6 4s2) in d-orbitals results into electron-electron repulsions which causes an increase in size. The increased nuclear charge makes the atom to shrink while electron-electron repulsions cause the size to expand. In Fe, these two opposing effects almost counter balance each other and, therefore, there is no change in size on going from Mn to Fe. In the last elements Zn ,Cd and Hg, these electron-electron repulsions become predominant and, therefore, there is increase in size.
(c) The atomic radii increase while going down the group. Therefore, the atomic radii of transition metals of second series have large values than those of the first transition series. However, the transition metals of third transition series have nearly the same radii as metals of second transition series above them.
Explanation: The atomic radii of elements of second transition series are more than those of first transition series because the electrons in the atoms of second transition series elements occupy energy levels farther from the nucleus. With the increase in number of outermost shell, size increases. The similar atomic radii of elements of second and third transition series are due to a special phenomenon known as lanthanoid contraction.
This is associated with the intervention of the 4f-orbitals which are filled before the 5d series of elements starts. The filling of 4f-orbitals before 5d orbitals results in regular decrease in atomic radii which compensates the expected increase in atomic size with increasing atomic number. As a result of this lanthanoid contraction, the elements of second and third transition series have almost similar atomic radii (e.g., Zr =160 pm, Hf =159 pm). Because of this, these elements of two series also have very similar physical and chemical properties. It is discussed later.
Ionic Radii of Transition Element
The ionic radii decrease with increase in oxidation state. For the same oxidation state, the ionic radii generally decrease with increase in nuclear charge.
Metallic Character and Enthalpy of Atomization
All the transition elements are metals. They all have high density, hardness, high melting and boiling points, high tensile strength, ductility, malleability, high thermal and electrical conductivity and lustre. With the exception of zinc, cadmium and mercury, the transition elements are much harder, less volatile and brittle than s-block elements. They have simple hexagonal close packed (hcp), cubic close packed (ccp) or body centred cubic (bcc), lattices which are characteristic of other metals. Mn, Zn, Cd and Hg, all the transition metals have one or more typical metallic structures at normal temperatures.
Explanation: The metallic character of transition elements is due to their relatively low ionisation enthalpies and number of vacant orbitals in the outermost shell.
These elements exhibit high enthalpies of atomization. The maxima at about the middle of each series indicates that one unpaired electron per d-orbital is favourable for strong inter-atomic interactions.The greater number of unpaired d-electrons, greater is the number of bonds and therefore, greater is strength of these bonds. Thus, as we move from left to right in a particular d-series, the number of unpaired electrons increases from 1 to 6 and then decreases to zero.
|3d 1 4s2||3d 24s2||3d 3 4s2||3d 5 4s1||3d 54s2|
|No. of unpaired electrons||1||2||3||6||5|
|3d 6 4s2||3d 7 4s2||3d 8 4s2||3d 10 4s1||3d 1 4s2|
|No. of unpaired electrons||4||3||2||1||0|
Density of Transition Element
All these metals have high density. Within a period, the densities vary inversely with the atomic radii. As we move in a period, the densities increase (as the radii decrease)
For example: The density decreases from titanium to copper.
However, the densities increase upon descending a group. The densities of second transition series are higher than those of first transition series and the densities of third transition series are still higher. Osmium (22.57 em) has the highest density among these elements.
Explanation: The atomic volumes of the transition elements are low because the electrons are added in (n-1) d-Subshell and not in ns-subshell. Therefore, the increased nuclear charge is partly screened by the d-electrons and the outer electrons are strongly attracted by the nucleus. Moreover, the added electrons occupy inner orbitals. Consequently, the densities of transition metals are high. Therefore, decrease in atomic radius coupled with increase in atomic mass results in general increase in the density of these elements.