(N/A) Hydrogen is the first element in the periodic table. Its electronic configuration is $1s^1$.
On one hand,its configuration is similar to the outer electronic configuration $ns^1$ of alkali metals (Group $1$),which lose one electron to form unipositive ions and form oxides,halides,and sulphides.
On the other hand,like halogens (Group $17$),it is short by one electron to achieve the nearest noble gas configuration.
However,unlike alkali metals,it has a very high ionization enthalpy $(\Delta_{i}H = 1312 \ kJ \ mol^{-1})$ and does not possess metallic characteristics under normal conditions.
Like halogens,it forms a diatomic molecule $(H_2)$ and covalent compounds,but its reactivity is much lower than that of halogens.
Furthermore,the loss of an electron from a hydrogen atom results in a nucleus $(H^+)$ of size $\sim 1.5 \times 10^{-3} \ pm$,which is extremely small compared to normal atomic and ionic sizes ($50$ to $200 \ pm$).
Consequently,$H^+$ does not exist freely. Due to this unique behavior,hydrogen is best placed separately in the periodic table.