Hydrogen :

(1) Position of hydrogen in the periodic table

Hydrogen is the first element in the periodic table. Hydrogen is placed in no specific group due to its property of giving electron (When H^– is formed) and also losing electron (When H⁺ is formed).

(i) Hydrogen is placed in group I(Alkali metals) as,

(a) It has one electron in its (Outer) shell-1s¹ like other alkali metals which have (inert gas) ns¹ configuration.

(b) It forms monovalent H⁺ ion like Li⁺, Na⁺ ..........

(c) Its valency is also 1.

(d) Its oxide (H₂O) is stable as Li₂O, Na₂O.

(e) It is a good reducing agent (In atomic as well as molecular state) like Na, Li ............

(ii) Hydrogen also resembles halogens (Group VII A) as,

(a) It is also diatomic (H₂) like F₂, Cl₂ ...........

(b) It also forms anion H^– like F^–, Cl^– by gain of one electron.

(c) H^– has stable inert gas (He) configuration as CH₄, C₂H₆ like halogens CCl₄, SF₂Cl₂ etc.

(d) H is one electron short of duplet (Stable configuration) like F, Cl which are also one electron deficient than octet, F – 2s²2p⁵; Cl – 3s²3p⁵.

(e) (IE) of H(1312 kJ mol^–1) is of the same order as that of halogens.

(iii) (IE) of H is very high in comparison with alkali metals. Also size of H⁺ is very small compared to that of alkali metal ion. H forms stable hydride only with strongly electropositive metals due to smaller value of its electron affinity (72.8 kJ mol^–1).

(iv) In view of the anomalous behaviour of hydrogen, it is difficult to assign any definite position to it in the periodic table. Hence it is customary to place it in group I (Along with alkali metals) as well as in group VII (Along with halogens).

(2) Discovery and occurrence : It was discovered by Henry Cavendish in 1766. Its name hydrogen was proposed by Lavoisier. Hydrogen is the 9^th most abundant element in the earth’s crust.

Hydrogen exists in diatomic state but in triatomicstate it is called as Hyzone. Systematic name of water is oxidane.