Q.Which is the first element in the periodic table ?

Hydrogen has the electronic configuration $1s^1$, a single electron in its valence shell, which gives it a dual, anomalous character. Like the alkali metals (Group 1) it can lose this one electron to form $H^+$, and it is often placed above lithium for this reason. Yet unlike the alkali metals it has a very high ionisation enthalpy ($1312\,kJ\,mol^{-1}$) because there is no intervening shell to shield the lone electron from the nucleus, so it does not lose the electron as readily as a true alkali metal. Hydrogen can equally well gain one electron to attain the stable duplet configuration of helium ($1s^2$), forming the hydride ion $H^-$, a behaviour shared with the halogens (Group 17), which also need exactly one electron to complete an octet. However, hydrogen's electron-gain enthalpy is far less negative than that of the halogens, and it does not form a stable diatomic $H_2^-$ analogue the way halogens form $X_2$. Because it resembles both groups but matches neither perfectly, hydrogen is best regarded as occupying a unique position of its own in the periodic table, sometimes shown in both Group 1 and Group 17, or set apart entirely.