question_answer 45)

Compare the general characteristics of the first series of transition metals with those of those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points : (i) electronic configurations, (ii) oxidation states, (iii) ionisation enthalpies and (iv) atomic sizes.

Answer:

(i) Electronic configurations. The elements the same vertical column generally have similar valence shell electronic configurations. However, the first series show only two exceptions, i.e., Cr= 3d54s1 and Cu = 3d10 4s1 but second series show more exceptions, e.g., Mo (42) = 4d55s1, Tc(43) = 4d65s1, Ru(44) = 4d75s1, Rh (45) =4d85s1, Pd(46) = 4d105s0, Ag (47) = 4d105s1. Similarly, in the third series, W (74) = 5d46s2, Pt(78) = 5d9, 6s1, and Au (49) = 5d10 6s1. (ii) Oxidation states. The elements in the same vertical column generally show similar oxdiation states. The number of oxidation states shown by the elements in the middle of each series is maximum and minimum at the extreme ends. The stability of higher oxidation state increases down the group. (iii) Ionization enthalpies. The first ionization enthalpies in each series generally increase gradually as we move from left to right though some exceptions are observed in each series. The first ionization enthalpies of some elements in the second (4d) series are higher while some of them have lower value than the elements of 3d series in the same vertical column. However, the first ionization enthalpies of third (5d) series are higher than those of 3d and 4d series. This is because of weak shielding of nucleus by 4f electrons in the 5d series. (iv) Atomic sizes. In general, ions of the same charge or atoms in a given series show progressively decrease in radius with increasing atomic number though the decrease is quite small. But the size of the atoms of the 4d series is larger than the corresponding elements of 3d series whereas those of corresponding elements of the 5d series nearly the same as those of 4d series because of lanthanoid contraction.