The elements in which electrons are progressively filled in $4f$ orbital are called

Assertion : Magnetic moment of $Dy^{3+}$ is highest among lanthanoids.
Reason : Orbital motion contributes to the magnetic moment.

Name the members of the lanthanoid series which exhibit $+4$ oxidation state and those which exhibit $+2$ oxidation state. Try to correlate this type of behavior with the electronic configurations of these elements.

Fill in the blanks:
$1.$ The size of $M^{3+}$ ions in the actinoid series ........... with an increase in atomic number.
$2.$ The common oxidation state of elements in the actinoid series is .......... .
$3.$ The appearance of elements in the actinoid series is like .......... .

Compare the chemistry of the actinoids with that of lanthanoids with reference to:
$(i)$ electronic configuration
$(ii)$ oxidation states and
$(iii)$ chemical reactivity.

What is lanthanoid contraction? What are the consequences of lanthanoid contraction?