Chemistry - f-block Question with Solution | TestHub

$a=15$: Only Promethium (Pm) is radioactive in lanthanides (1), and all actinides (14 elements from Th to Lr) are radioactive ($1+14=15$).

$b=0$: Europium (Eu) has the largest atomic radius among all lanthanides due to its stable half-filled $f^7$ configuration, so no element is larger.

$c=6$: The highest stable oxidation state exhibited by Americium (Am) is +6 (found in species like $Am{O}_2^{2+}$).

$d=3$: Only three actinides occur naturally in significant amounts: Thorium (Th), Protactinium (Pa), and Uranium (U).

Calculation: Substituting these values into the expression gives $(a+b-c-d)=(15+0-6-3)=6$.