Question

Lanthanum is grouped with f-block elements because
(A) it has partially filled f-block orbitals
(B) it is just before Ce in the periodic table
(C) it has both partially filled f and d-orbitals
(D) the properties of lanthanum are very similar to the elements of $4f$ block

Answer 1

Lanthanides are f-block elements. Their atomic number ranges from $57$ to $71$ . They are also called rare earth metals. They have similar configurations and similar physical and chemical behavior. They are relatively soft metals. Silvery-white metals that tarnish when exposed to air, forming their oxides.

Complete answer:
The lanthanide family contains fifteen metallic elements (from lanthanum to lutetium). The valence electrons of the lanthanide family are present in the 4f orbital. All of them are known to form Ln3+ cations. Lanthanides show variable oxidation states. They also show +2, +3, and +4 oxidation states. Among them, the most stable oxidation state of Lanthanides is +3.
The general electronic configuration of lanthanides is $\left[ {Xe} \right]{\text{ }}4{f^{1 - 14}}{\text{ }}5d{{\text{ }}^{0 - 1}}6{s^2}$ .
In lanthanide the electron enters in 6s orbital therefore lanthanides are placed in period number 6 and group number $3$ .
The energies of $4f$ and $5d$ electrons are almost close to each other and so $5d$ orbital remains vacant and the electrons enter into the $4f$ orbital. Also, they have partially filled f-orbitals. F-orbitals of electron shells are filled with $n{\text{ }}-{\text{ }}2$ electrons. This is the reason why lanthanum is grouped with f-block elements.
Therefore the correct option is A. it has partially filled f-block orbitals.

Note:
Lanthanide name is given after their first element of the series lanthanum. They all show similar properties. With the increase in the atomic number there is a decrease in the size of the atoms this is called lanthanide contraction. It is only shown by lanthanides.