Explain the size of actinoid elements and their ions.

(N/A) $(i)$ In the actinoid series, the size of atoms and $M^{3+}$ ions decreases gradually with an increase in atomic number.
$(ii)$ Actinoid Contraction: The gradual decrease in the size of atoms and ions of actinoids is known as actinoid contraction. This contraction occurs because of the poor shielding effect of $5f$ electrons, which increases from one element to the next.
$(iii)$ The radii (in $pm$) of some $M^{3+}$ and $M^{4+}$ ions are given in the table below:

Atomic Number $(Z)$	Name/Symbol	$M^{3+}$ $(pm)$	$M^{4+}$ $(pm)$
$89$	Actinium $(Ac)$	$111$	-
$90$	Thorium $(Th)$	-	$99$
$91$	Protactinium $(Pa)$	-	$96$
$92$	Uranium $(U)$	$103$	$93$
$93$	Neptunium $(Np)$	$101$	$92$
$94$	Plutonium $(Pu)$	$100$	$90$
$95$	Americium $(Am)$	$99$	$89$
$96$	Curium $(Cm)$	$99$	$88$
$97$	Berkelium $(Bk)$	$98$	$87$
$98$	Californium $(Cf)$	$98$	$86$

$(iv)$ These elements are radioactive, making their study difficult.