Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).…
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) :
Ionisation enthalpies of early actinoids are lower than for early lanthanoids.
Reason (R) :
Electrons are entering $5 f$ orbitals in actinoids which experience greater shielding from nuclear charge.
In the light of the above statements, choose the correct answer from the options given below:
Both $(\mathbf{A})$ and $(\mathbf{R})$ are true and $(\mathbf{R})$ is the correct explanation of $(\mathbf{A})$.
Both $(\mathbf{A})$ and $(\mathbf{R})$ are true but $(\mathbf{R})$ is not the correct explanation of $(\mathbf{A})$.
(A) is true but (R) is false.
(A) is false but $(\mathbf{R})$ is true.
Solution
Ionisation enthalpies of early actinoids are lower than early lanthanoids. This is because $5 f$ orbitals will penetrate less into the inner core of electrons. The $5 f$ electrons, will be more effectively shielded from the nuclear charge than the $4 f$ electrons of corresponding lanthanoids.