Q.Write the formulae of the compounds of hydrogen formed with sodium and chlorine.

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 les …