Question

Which element exhibits anomalous behaviour so that it can be placed in two different groups of the periodic table?

Answer 1

The element hydrogen ($\text{H}$) can form hydrogen halides meaning, it can form ${\text{H}}^{+}$ ions in $\text{HF, HCl, HBr}$, etc. It resembles group-I elements like lithium, sodium, potassium as they form ${\text{Li}}^{+},{\text{Na}}^{+},{\text{K}}^{+}$ in metal halides ($\text{LiCl, NaCl, KBr}$). So, it can be placed in group-I along with $\text{Li, Na, K}$.

Hydrogen also resembles halogens as it can form hydrides. It can form ${\text{H}}^{-}$ ions in $\text{NaH, LiH,}$ etc. Halogens can form such halide ions in NaF, NaCl, KBr $({\text{F}}^{-},{\text{Cl}}^{-},{\text{Br}}^{-}$). So, hydrogen can also be placed in ${17}^{th}$ group along with F, Cl, Br, I as it resembles halogens also. This behavior of hydrogen makes it difficult to place it in a single group.