Question

Give reason:
(a)Cerium ${\text{(Ce)}}$ exhibits $+ 4$ oxidation states.
(b)Actinoid contraction is greater from element to element than lanthanoid contraction.

Answer 1

The oxidation state is determined by the number of electrons gained or lost by an atom to complete its octet. The contraction is nothing but the reduction in size of the atoms, contraction is seen in lanthanoids and actinoids due to poor shielding of ${\text{4f and 5f}}$ electrons.

Complete step by step answer:
(a)Cerium ${\text{(Ce)}}$ exhibits $+ 4$ oxidation states.
Cerium is a lanthanide series element. The atomic number of cerium is 58 and it is a soft, white- silvery element.
The oxidation state of an element is defined as the number of electrons gained or lost by an atom in order to form chemical bonds. So we will try to find out how many electrons it can gain or lose to complete its octets. The simple method to find the number of electrons in an atom is electronic configuration, hence the electronic configuration of cerium is:
$\left[ {{\text{Xe}}} \right]4{{\text{f}}^{\text{2}}}{\text{5}}{{\text{d}}^{\text{0}}}{\text{6}}{{\text{s}}^{\text{2}}}$
So to achieve the inert gas configuration it can lose the outermost four electrons present in ${\text{s and f}}$ orbitals.
$\left[ {{\text{Xe}}} \right]4{{\text{f}}^{\text{0}}}$ is the electronic configuration of nearest noble gas.
Therefore the oxidation state of cerium ${\text{(Ce)}}$ is $+ 4$ .
(b)Actinoid contraction is greater from element to element than lanthanoid contraction.
Contraction is nothing but the process of becoming smaller or reduction in the size of something. Here we are talking about the lanthanoids and actinoids.
The actinoid contraction is greater than the lanthanoid contraction because the shielding of $5f$ electrons in actinoids is poor as comparable to $4f$ electrons in lanthanides. This poor shielding in actinoids results in higher effective nuclear charge to the outer electrons, hence the electrons come closer to the nucleus due to attraction and thus the atomic size is reduced.
Therefore,
Actinoid contraction is greater from element to element than lanthanoid contraction.

Note:
Lanthanoids and Actinoids are $f{\text{ block}}$ elements, these are separated from the mainstream as they show different periodicity of electronic structures. These are placed separately in periodic tables as the electrons are filled in ${\text{4f and 5f}}$ orbitals.