The d-And-f-Block Elements

Question

# Explain each of the following observations:(i) With the same d-orbital configuration (d4), Cr2+ is a reducing agent while Mn3+ is an oxidising agent.(ii) Actinoids exhibit a much larger number of oxidation states than the lanthanoids.(iii) There is hardly any increase in atomic size with increasing atomic number in a series of transition metals.

(i)  Outer electronic configuration f Mn is 3d5 4s2.

Outer electronic configuration of Mn3+ is 3d4 4s0.

Now Mn3+ is a strong oxidizing agent. A good oxidizing agent reduces itself. I.e. gains electrons from other. Its tends to gain one more electron to acquire stable electronic configuration. If it gains one electron, its configuration will be 3d5, which is stable .this is the reason, it acts as a good reducing agent.

Cr2+ is strongly reducing agent in nature. It has d4 configuration. While acting as a reducing agent, it gets oxidized to Cr3+ such as d3. This d3 configuration can be written as t32g configuration, which is more stable configuration.

(ii) In actinoids, the 5f, 6d, 7s shells are present. These three shells are of comparable energies; therefore electrons can remove from these shells. This gives rise f variable oxidation states in actinoids.

Following are the similarities between actinoids and lanthanoids:

(a) The size of atom: It decreases across the series in both actinoids (due to actinoid contraction) and lanthanoids (due to lanthanoids contraction).

(b) Oxidation states: Lanthanoids and actinoids generally show +3 oxidation states. However, some element in the actinoids series is capable of exhibiting oxidation states higher than +3

(iii) The atomic sizes of the elements of the first transition series are smaller than those of the heavier elements (elements of 2nd and 3rd transition series).

However, the atomic sizes of the elements in the third transition series are virtually the same as those of the corresponding members in the second transition series. This is due to lanthanoid contraction.