Question

What is lanthanide contraction? Explain the cause of lanthanide contraction.

Answer 1

Lanthanide contraction gradually decreases in the atomic and ionic size of lanthanide with an increase in atomic number. As the atomic number increases, positive charge on the nucleus increases and the valence shell contracts. In a periodic table, On going down the group the atomic size first increases but when it comes to f-block elements this trend does not apply anymore. That decrease in atomic radius of the f-block element in a group is called lanthanide contraction.

Complete step by step answer:
The lanthanide contraction is the greater than expected decrease in ionic radii of the element in the lanthanide series from atomic number $57$ , lanthanum $71$ , lutetium which result in smaller than otherwise expected ionic radii for the subsequent element starting with $72$ , hafnium.
Cause: - When there is an increase in atomic number, the positive charge on the nucleus also increases by one unit and one more electron also enters the same 4f-subshell.
Generally, a shielding effect operates when electrons are added in outer shells, electrons are already present that shield the outer electrons from nucleus charge. They make them experience a lower effective charge on the nucleus. The shielding effect exerted by the inner electrons decreases in the following order s>p>d>f.
When the charge on the nucleus increases, the valence shell is pulled slightly towards the nucleus. In simple words, the valence shell contract. This causes lanthanide contraction.

Note:
When the atomic number increases, the number of subshells increases. But make it clear that the atomic radius does not increase, that is the atomic radius decreases this effect is called lanthanide contraction. This effect is always shown in f-block elements.