The trivalent ion having the largest size in the lanthanide series is:

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ All actinoid elements have a $7s^1$ configuration.
$(b)$ All actinoid elements have a $7s^0$ configuration.
$(c)$ There are no electrons in the $7s$ orbital of actinoids.

Fill in the blanks:
$1.$ In the elements of the actinoid series,the last electron enters the ........... orbitals.
$2.$ Inner transition elements are the elements of the .......... series.
$3.$ If electrons are filled in the $5f$ series,those elements are called ....... elements.
$4.$ Many elements of the actinoid series are ......... .

Explain the lanthanoid contraction.

Which of the following elements belongs to the actinoid series?

State the consequences of lanthanoid contraction. Explain the formation of coloured ions by lanthanoids.