There is a regular decrease in the atomtextc and ionic radii of lanthanide from $\text{L}\text{a}$ to $\text{L}\text{u}$ . This decrease in radii is a unique feature and is known as lanthanide contraction. It is due to greater effect of the increased nuclear charge than that of the screening effect. As a result of lanthanide contraction the normal increase in size from $\text{S}\text{c}\to \text{Y}\to \text{L}\text{a}$ disappears after the lanthanide series and the pair of elements like $\text{Z}\text{r}$ and $\text{H}\text{f}$ have almost the same radii.