$(a)$ State Modern Periodic Law.
$(b)$ Elements $A, B, C$ and $D$ have atomic numbers $1, 8, 11$ and $19$ respectively. Choose the odd element and give reason for your answer.

(B) The Modern Periodic Law states that the properties of elements are a periodic function of their atomic numbers.
$(b)$ The odd element is $B$ (atomic number $8$).
Reason: The electronic configuration of $A$ $(Z=1)$ is $1$,$C$ $(Z=11)$ is $2, 8, 1$,and $D$ $(Z=19)$ is $2, 8, 8, 1$. All these elements $(A, C, D)$ have $1$ electron in their valence shell and belong to Group $1$ (alkali metals). However,$B$ $(Z=8)$ has an electronic configuration of $2, 6$,meaning it has $6$ electrons in its valence shell and belongs to Group $16$.