Write a short note on lanthanide contraction.

Hint: Lanthanides start from atomic number 57 (Lanthanum) to atomic number 71 (Lutetium), as it is placed as separate below the transition metal elements, so it is named as lanthanoids. They have a special effect called lanthanide contraction which depends on the nuclear charge and shielding effect which has consequences on the atomic size of the second and third transition elements series.

Complete answer:
We know that there are many elements in the periodic table and they are divided into four parts, i.e, s-block elements, p-block elements, d-block elements, and f-block elements.
The d-block elements contain lanthanoids and actinoids which are placed below them because they are f-block elements. Lanthanides start from atomic number 57 (Lanthanum) to atomic number 71 (Lutetium), as it is placed as separate below the transition metal elements, so it is named as lanthanoids.
The atomic size of the elements of the second and third transition series is nearly the same because of the lanthanoid contraction because the lanthanoids come in the third series. As we know that when the electrons are added into the same shell the nuclear charge increases and size decreases. This happens in lanthanide because as the atomic number increases, the nuclear charges increase by one unit, so it is expected that the atomic size will decrease but the electrons are added into the f-orbital and the electrons of the f-orbital have a very poor shielding effect. Which counterbalances the nuclear charge and there are very small decreases in size. This is known as lanthanoid contraction.
Due to this contraction, the size of the second and third transition elements series are very similar.

Note:
This poor shielding effect is also seen in the d-orbitals because the electrons in the d-orbital have a poor shielding effect. This shielding effect is lowest in the s-orbital.