frequent metal – metal bonding in compounds of the heavy transition metals. Fig. . Trends in enthalpies of atomisation of transition elements In general, ions of the same charge in a given series show progressive decrease in radius with increasing atomic number.
This is because the new electron enters a d orbital each time the nuclear charge increases by unity. It may be recalled that the shielding effect of a d electron is not that effective, hence the net electrostatic attraction between the nuclear charge and the outermost electron increases and the ionic radius decreases. The same trend is observed in the atomic radii of a given series. However, the variation within a series is quite small.
An interesting point emerges when atomic sizes of one series are compared with those of the corresponding elements in the other series. The curves in Fig. . show an increase from the first ( d ) to the second ( d ) series of the elements but the radii of the third ( d ) series are virtually the same as those of the corresponding members of the second series.
This phenomenon is associated with the intervention of the f orbitals which must be filled before the d series of elements begin. The filling of f before d orbital results in a regular decrease in atomic radii called Lanthanoid contraction which essentially compensates for the expected . . Variation in Atomic and Ionic Sizes of Transition Metals D a H V /kJ mol – increase in atomic size with increasing atomic number.
The net result of the lanthanoid contraction is that the second and the third d series exhibit similar radii (e.g., Zr pm, Hf pm) and have very similar physical and chemical properties much more than that expected on the basis of usual family relationship. The factor responsible for the lanthanoid contraction is somewhat similar to that observed in an ordinary transition series and is attributed to similar cause, i.e., the imperfect shielding of one electron by another in the same set of orbitals. However, the