Hydrogen has one electron which it can either lose or gain or share to acquire noble gas, i.e., helium gas configuration.

Therefore, in principle, it can form either ionic or covalent bonds. But the ionisation enthalpy of hydrogen is very high ( $1312 \mathrm{~kJ} \mathrm{~mol}^{-1}$ ) and its electron gain enthalpy is only slightly negative $(-73 \mathrm{~kJ} \mathrm{~mol}^{-1})$.

From this consequence, it does not have a high tendency to form ionic bonds but rather prefers to form only covalent bonds.

The ionisation enthalpy of hydrogen higher than that of sodium. Both hydrogen and sodium have one electron in the valence shell. But the size of hydrogen is much smaller than that of sodium and hence, the ionisation enthalpy of hydrogen is much higher $\left(1312 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ than that of sodium $\left(496 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)$.

Basic principle of hydrogen economy is transportation and storage of energy in the form of liquid or gaseous hydrogen. Hydrogen is a gas at room temperature.

However, by cooling and applying high pressure, gaseous $\mathrm{H}_2$ can be converted into liquid $\mathrm{H}_2$ which has much smaller volume and hence can be transported easily. Thus, the basic property of hydrogen which is useful for hydrogen economy is that it can be converted into a liquid by cooling under high pressure.

Following are the importance of heavy water

(i) It is extensively used as a moderator in nuclear reactors.

(ii) It is used as a tracer compound in the study of reaction mechanism.

(iii) It is used for the preparation of other deuterium compounds such as $\mathrm{CD}_4, \mathrm{D}_2 \mathrm{SO}_4$, etc.

The Lewis structure of hydrogen peroxide is