Why can hydrogen be placed in either group 1 or group 7 ?

Hydrogen resembles alkali metals i.e. Li , K , Na , K, Rb ,Cs and Fr of group 1 of the periodic table.

1) Electronic configuration : Like alkali metals, hydrogen also contains 1 electron in its outermost shell.

Hydrogen 1s1

Lithium 1s2 2s1

Sodium 1s2 2s2 2p6 3s1

2) Electropositive character: Like alkali metals ,hydrogen also loses its only electron to form hydrogen ion i.e. H+

H ——-> H+ + e‾

Na ———-> Na+ + e‾

Hydrogen like alkali metals exhibit electropositive character.

3) Oxidation State:  Like alkali metals, hydrogen exhibits an oxidation state of  +1 in its compounds.

H+ Cl‾

Na+ Cl‾

4) Combination with electronegative elements or non metals: Like alkali metals, hydrogen combines with electronegative elements such as oxygen ,halogen and sulphur forming their oxides ,halides and sulphide.

Oxides H2O like Na2O , K2O

Halides HCl like NaCl , KCl

Sulphides H2S like Na2S , K2S

5) Liberation at the cathode : When an aqueous solution of HCl is electrolysed H2 is liberated at the cathode in the same way as alkali metals are liberated at cathode during the electrolysis of their fused halides.

6)Reducing character: Like alkali metals, hydrogen also act as a strong reducing agent.

Fe2O3 + 4 H2 ———> 3Fe + 4H2O

B2O3 + 6 K ————–> 2B + 3 K2O

SIMILARITY with halogens

Hydrogen resemble halogens i. F , Cl, Br , I of group 17 of the periodic table in the following ways:

1) Electronic configuration: All the halogens have 7 electron in their respective outermost shell and thus have one electron less than the stable configuration of the nearest inert gas. Hydrogen has one electron in the outermost shell and thus has one electron less than the stable configuration of the nearest inert gas i.e. helium.

H 1s1 one electron less than He

F 1s2 2s2 2p5 one electron less than Ne 1s2 2s2 2p6

2) Electronegative character: Halogens have a strong tendency to gain one electron to form halide ions. Hydrogen show some tendency to gain one electron to form hydride ions.

H + e‾ ————-> H‾

Cl + e‾ —————-> Cl‾

3) Ionization enthalpy : Ionization enthalpy of hydrogen is quite compatible with those of halogens but much higher than those of alkali metals.

4) Oxidation State: Just like halogens, hydrogen shows an oxidation state of -1.

5)Liberation at the anode:  When fused alkali metal hydrides such as Lithium, sodium hydride is subjected to electrolysis ,hydrogen is liberated at the anode. Similarly halogens are liberated at the anode when fused alkali metal halides are electrolysed.

2NaH ( l ) ————> H2 ( g ) + 2Na ( l )

2 NaCl ( l ) ————–> Cl2 ( g ) + 2Na ( l )

6) Atomicity and non metallic character: Just like halogens, hydrogen also exist as a diatomic molecule. Like halogens, hydrogen is a typical non metal.

7) Combination with metals: Hydrogen combines with highly electropositive alkali and alkaline earth metals to form metallic hydrides. Halogens combine with these metals to form metallic halides.

2Na + H2 ——-> 2NaH

Ca + H2 ——–> CaH2

8) Formation of covalent compounds: Hydrogen readily combines with non-metals such as carbon, Silicon ,nitrogen to form covalent compounds.

CH4 , SiH4 , NH3 , CCl4 , SiCl4

9) Replacement or substitution reaction:  In many compounds of carbon ,hydrogen can be replaced by halogens and halogens can be replaced by hydrogen.

CH4 + Cl2 ——-> CH3Cl + HCl

CH3Cl + 2 [H] ——–> CH4 + HCl

this means that the temperature of both substances will be equal