Number of electrons present in $4f$ orbital of $Ho^{3+}$ ion is $.....$
(Given Atomic No. of $Ho=67$)

Identify the incorrect statement in the following.

State whether the following statements are true or false:
$(i)$ Atomic size increases with an increase in the atomic number of elements.
$(ii)$ The electron gain enthalpy for noble gases is approximately zero.
$(iii)$ The first element of the actinoid series is $Th$ and the last element is $Lu$.
$(iv)$ The general electronic configuration of $d$-orbitals is always $(n-1)d^{1-10}ns^2$.

Which of the following factors may be regarded as the main cause of lanthanide contraction?

Transition metals are related to which block?

Which of the following statements is not true about interstitial compounds?