In f-block elements, the atomic radius almost remains same because of the _____.

The most basic hydroxide from following is ____.

Because of lanthanide contraction, which of the following pairs of elements have nearly same atomic radii ? (Numbers in the parenthesis are atomic numbers)

Which of the following pairs has the same size?

Identify a $‘\mathrm{chemical} \mathrm{twin}’$ among the following.

Which of the following statements is INCORRECT to pair of elements $\mathrm{Zr}-\mathrm{Hf}$?

Which of the following is not an actinide?

The actinoids exhibit more number of oxidation states, greater than the lanthanoids. This is because ______.

The reason for the greater range of oxidation states in actinoids is attributed to _____.