I) a) Account for the following:

i) Zr and Hf have almost similar atomic radii

ii) Transition metals show variable oxidation states

iii) ion is unstable in an aqueous solution

b) Complete the following equations:

II) a)

From the given data of values, answer the following questions:

i) Why is value exceptionally positive?

ii) Why is value highly negative as compared to other elements?

iii) Which is stronger reducing agent ? Give reason.

b) Why do actinoids show a wide range of oxidation states? Write one similarity between the chemistry of lanthanoids and actinoids.

I) a) (i) Zr and Hf have almost similar atomic radii because of lanthanoid contraction. This can be explained by the imperfect shielding of one electron by another in the same sub shell. However, the shielding of one 4 f electron by another is less than one d electron by another with the increase in nuclear charge along the series. There is fairly regular decrease in the sizes with increasing atomic number.

ii)Transition metals show variable oxidation states because there is a very small energy difference in between (n-1)d and ns orbitals. As a result, electrons of (n-1)d orbitals as well as ns-orbitals take part in bond formation.

(iii) ion is unstable in aqueous solutionwhile is stable because removal of one electron from is required to make it ion, this energy requirement is stabilised by the hydration energy thereby making it unstable in an aqueous solution and gives ion and Cu (s).

b)i)

ii

(II)

a)i) There are many factors which influence the reduction potential of any element which include its electronic configuration, ionisation enthalpy and hydration enthalpy etc. In case of Copper, its ionisation enthalpy is so high which is compensated by the hydration energy therefore, it has exceptionally positive potential.

ii) value of Manganese ion is highly negative as compared to other elements, it is due to the fact that it has very high enthalpy of atomization and ionisation, this is the reason it is not compensated by the hydration enthalpy.

iii) is stronger reducing agent than . It is because the electronic configuration of Fe is while the EC of . Now, the EC of Cr is and ion is Thus, is stronger reducing agent as it gets partially filled octet or half-filled electronic configuration which makes it more stable.

(b) Actinoids show a wide range of oxidation state which is in part attributed to comparable energies of 5f, 6d and 7s levels. The actinoids show in general +3 oxidation state. The elements, in the first half of the series frequently exhibit higher oxidation states. For example, the maximum oxidation state increases from +4 in Th to +5, +6 and +7 respectively in Pa, U and Np but decreases in succeeding elements. The actinoids resemble the lanthanoids in having more compounds in +3 state than in the +4 state. Similarity between lanthanides and actinides is that both have a prominent oxidation state of +3 and are involved in the filling of (n-2) f orbitals.