Interstitial hydrides have metallic bonding. These are non - stoichiometric; their composition varies with temperature and pressure. The crystal lattice expands due to the inclusion of dihydrogen and there is distortion of crystal lattice. So they have lower density than the parent metal. Recent studies have shown that except the hydrides of Ni, Pd, Ce & Ac, other hydride are different from the parent metals.

a) HCl is a gas. It is an electron rich covalent hydride. It is a covalent molecule with strong covalent bond. However the forces between the molecules are not as strong.
b) NaH is a solid. It is an ionic hydride of group I sodium metal. It is an insoluble hydride

Hydrogen has electronic configuration 1s¹. So it resembles ns¹ general valence shell configuration of alkali metals.
Its similarity with alkali metals are:
1. Form uni positive (H⁺) ions like Na⁺, K⁺, Cs⁺
2. Form halides, (HX), oxides (H₂O) peroxides (H₂O₂) and sulphides (H₂S) like (Nax, Na2O, Na₂O₂, Na₂S)
3. Acts as reducing agent. 
4. Has +1 oxidation state.
5. If it is placed in the halogen family it should be the most electro negative element which is not true.

NOTE:
At any condition ice cube does not sink in water.
At 0⁰C, ice and liquid water will be in equilibrium and will coexist. Hence no freezing or melting will occur.

(i) Due to small size of Nitrogen atom.
(ii) Due to polar nature of N-H bond.
(iii) Due to inter molecular H- bonding which are stronger than London forces present in other hydrides. Other hydrides lack H - bonding