$(a)$ Both helium and beryllium have two electrons in their valence shells. Helium is a noble gas whereas beryllium is a metal. Justify.
$(b)$ Hydrogen exists in three isotopic forms. Why are the isotopes of hydrogen chemically alike?

(N/A) 'He' has atomic number $2$,so it has $2$ electrons in the $K$-shell,which is the first energy level and has a maximum capacity of $2$ electrons. Thus,its valence shell is completely filled,making it a noble gas. Conversely,'Be' has an atomic number of $4$ and an electronic configuration of $2, 2$. It has $2$ electrons in the $L$-shell,which has a capacity of $8$ electrons. Since its valence shell is incomplete,'Be' acts as a metal with a valency of $2$.
$(b)$ Hydrogen exists in three isotopic forms: protium $({ }_{1}^{1} H)$,deuterium (${ }_{1}^{2} H$ or $D$),and tritium (${ }_{1}^{3} H$ or $T$). All three isotopes have the same atomic number $(1)$,meaning they possess the same electronic configuration ($1$ electron in the $K$-shell). Since chemical properties of an element are determined by its electronic configuration,all isotopes of hydrogen exhibit identical chemical behavior.