Answer the following:

(a)   Explain, why density of transition elements increases from left to right in a period?

(b)   Why E° values for Mn, Ni and Zn are more negative than expected?

(c)   What are inner-transition elements? Describe which of the following are the atomic numbers of the inner-transition elements: 29, 59, 74?

(d)   Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number?

Name the two transition elements which have abnormal electronic configuration and explain why?

(a) In transition metals, density increases as a result of the radii remains similar from left to right in a period but their atomic masses increases.

 (b) Negative values are related to the stabilities of Mn²⁺ half-filled and Zn²⁺ completely filled configuration respectively.

E° value is related to the highest negative enthalpy of hydration in Ni²⁺.

 ${\mathrm{E}}^{\circ }$ values for Mn, Ni and Zn are more negative than expected.

(c)   f-block elements are inner-transition elements that have incomplete f-orbital in the penultimate shell of their atoms which include

i.                 Lanthanoids (Z=58 to 71) and

ii.                Actinoids (Z = 90 to 103).

29 = [Ar] 4s¹ 3d¹⁰ (d-block element or transition element).

59 = [Ar] 6s¹ 4f³ (d-block element or inner transition element).

74 = [Xe] 6s² 4f¹ 3d³ (d-block element or inner transition element).

(d)    Elements that have more stable +2 oxidation states are ₂₁Sc²⁺, ₂₂Ti²⁺, ₂₃V²⁺, ₂₄Cr²⁺ and ₂₅Mn²⁺.

The outer electronic configurations are 3d¹, 3d², 3d³, 3d⁴ and 3d⁵ respectively for the above-given elements.

If two electrons are removed from 4s, then the 3d-orbitals are occupied. So, the number of empty d-orbitals decreases.

The number of unpaired electrons in 3d-orbitals increases with an increase in the stability of the cations (M²⁺) increases from Sc to Mn.

(e)   Exceptions arise due to an electronic configuration having very little energy difference between (n-1) d and ns orbitals such as

Cr: 3d⁵ 4s¹ instead of 3d⁴ 4s²

Cu: 3d¹⁰ 4s¹ instead of 3d⁹ 4s²

This phenomenon occurs due to electronic repulsion and energy exchange reason for half-filled and completely filled d-orbital are more stable configuration.

In the transition elements, d-orbitals lie at the periphery of an atom which is why they are more influenced by the surroundings which affect the atoms or molecules surrounding them.