Among landthanoids, Ln(III) compounds are predominant. however, occasionally in solutions or in solid compounds, +2 and +4 ions are also obtained. Give reason.

1

(a)

Lanthanoids show +3 oxidation state mostly as 2 electrons from outer 6s orbital and one electron from 5d orbital take part in bond formation. Some show +2 and +4 oxidation states due to stablity of half filled and completely filled 4f orbitals.

There is hardly any increase in atomic size with increasing atomic numbers in a series of transition metals. Give reason.

1

(a)

It is because effective nuclear charge does not increase appreciably as pairing of electrons in d-orbitals take place which cause repulsion after Mn(25).

Complete and balance the following chemical equation Cr₂O₇^2-+I^-+H⁺ \(\longrightarrow \) _________________.

1

(a)

  Cr₂O₇^2-+6I^-+14H⁺ \(\longrightarrow \)2Cr³⁺+7H₂O+3I₂

What is meant by 'lanthanoid contraction'?

1

(a)

The decrease in size with the increase in atomic number among lanthanoids is called lanthanoid contraction.

Write the outer electronic configuration of Cr atom (Z=24).

1

(a)

Cr(24): [Ar] 3d⁵ 4s¹