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.

Hydrogen peroxide is highly unstable and the aqueous solution spontaneously disproportionates to give oxygen and water. The reaction is, however, slow but is explosive when catalyzed by metal. If it is stored in glass container, it dissolves the alkali metals from the glass, which catalyzes the disproportionation reaction. For this reason, H₂O₂ solutions are stored in plastic bottles.
H₂O₂ ⟶ H₂O + 1/2 O₂

Hydrogen peroxide can be prepared by adding a metal peroxide to dilute acid
BaO_2(s) + H₂SO_4(aq) ⟶ BaSO_4(s) + H₂O_2(aq)

In metallurgy, hydrogen can be used to reduce many metal oxides to metals at high temperatures.
\(CuO+H_2\rightarrow Cu+H_2O\)
\( WO_3+3H_2\rightarrow W+3H_2O\)

They form heavy water and halides.
\(2D_2+O_2\rightarrow2D_2O \)
\(D_2+X_2\rightarrow 2DX[X=F,C1,Br and I]\)

Hydrogen itself acts as a reducing agent. In the presence of finely divided nickel, it adds to the unsaturated organic compounds to form saturated compounds.
\(HC=CH\xrightarrow{Ni / H_2}H_2C=CH_2\xrightarrow{Ni/H_2}H_3C-CH_3\)

Ice has highly ordered 3D hydrogen bonded structure in which oxygen atom is tetrahedrally surrounded by four hydrogen atoms. This gives ice an open cage like structure. Density of ice is less than water. So ice floats in water.

Halogens react with water to give an acidic solution. For example, chlorine forms hydrochloric acid and hypochlorous acid.
Cl₂(g) + H₂O(l) ➝ HCl (aq) + HOCl(aq)

Steam when passed over hot iron gives iron oxide with the evolution of Hydrogen.
3Fe(s) + 4H₂O(l) ➝ Fe₃O₄(s) + H₂(g)

In the large-scale, hydrogen is produced currently by steam-reforming of hydrocarbons. That is, reaction of steam and hydrocarbons in the presence of a catalyst (nickel) at high pressure (35 atm.) and temperature (800°C).
CH₄(g)+H₂O(g) ➝ CO(g)+3H₂(g)

Electrolysis of heavy water gives deuterium.
\({ 2D }_{ 2 }O\overset { Electrolysis }{ \longrightarrow } { 2D }_{ 2 }+{ O }_{ 2 }\)

₁H₂ or ₁D₂ . It occurs naturally in very small traces. It's nucleus consists of a proton, a neutron and one electron revolving around the nucleus. Its chemical properties are similar to those of protium but their reaction rates are different.

Hydrogen molecule in which protons in the nuclei of both H-atoms spin in opposite direction is termed as parahydrogen.

a) Electron-precise hydrides:
These have required number of electron to represent their conventional d lewis structure. All the elements of carbon group (14) form such hydrides.
Eg CH₄, C₂H₆, SiH₄, GeH₄
b) Electron - deficient hydrides:
There act as Lewis acids (ie) electron acceptors. The elements from group (13) form such hydrides.
Eg: B₂H₆
c) Electron - rich hydrides:
There have excess electron, which are present as lone pairs. Elements of group 15-17 form such hydrides. They behave as Lewis bases (ie) electron donors.
Eg: NH₃ ,H₂O, HF

(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