Question

An element ${}^{\prime }{X}^{\prime }$ on emitting an $\alpha$ - particle, do not change its group number in the periodic table. The element is:

• A. lanthanum
• B. cerium
• C. lawrencium
• D. radium

Answer 1

The correct option is C lawrencium

Lawrencium $-[Z=103]-\left[R_n\right]{5f}^{14}{7S}^2{7P}^1$

emits $\alpha -$particles $\Rightarrow$ emits 2 electrons $\left[{-}_2^{4}He\right]$

It is in the f-block, on emitting also it still remains in f-block $\Rightarrow$ no change in group number.

whereas cerium, 1 anthanum and radium are emitting $\alpha -$particles and than go into $S$ and p-blocks.