1) Hydrogen is the first element in the periodic table.
2) It is the lightest element known.
3) It atomic form exists only at high temperature.
4) It exist as a diatomic molecule i.e. H2 .That is why it is also called dihydrogen.
5) It was discovered by Henry Cavendish in 1766.
6) Its electronic configuration is 1s1
Unique position of Hydrogen in periodic table
A proper position could not be assigned to it because of the following reason:
1) In some properties it resembles alkali metals. As such, it can be placed in group 1 of the periodic table along with alkali metals.
2) In some other properties, hydrogen resembles halogens. As such, it can be placed in group 17 along with halogens.
3) In some hydrogen differs from both alkali metals and halogens.
Resemblance with alkali metals
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.
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.
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
Resemblance 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
Difference from Alkali metal and halogens
1) Hydrogen is less electropositive than alkali metals and less electronegative than halogens.
Hydrogen has less tendency to form H+ ions as compared to alkali metals which readily form monovalent cation. Hydrogen has less tendency to form H‾ ions as compared to halogen which readily form halide ions(X‾) ions.
2) Unlike halogens and alkali metals, hydrogen contains only one proton in its nucleus and only one electron in the extranuclear part.
3) Nature of oxides : The nature of oxide of alkali metals ,halogens and hydrogen is quite different.
The oxides of alkali metals are basic while those of halogens are acidic but the oxides of hydrogen i.e. water is neutral.
4) Absence of unshared pair of electrons: The hydrogen atoms in hydrogen molecule do not possess any unshared pair of electron whereas each halogen atom in halogen molecule possesses three unshared pairs of electrons.
5) Nature of compounds : The compounds of hydrogen with halogen are low boiling covalent compounds whereas alkali metal halides are high melting ionic solids.
6) Size of ions :The size of H+ ion is much smaller than those of alkali metal cations but the size of H‾ ion is much larger than any of the halide ion.
7) Structure of hydrates: Being very small in size , H+ does not exist freely and is always associated with other atoms or molecules.
H+ ion exist in aqueous solution and hydrated proton having the formula [H9O4]+ in which 4 molecules are tetrahedrally arranged around the H+ ion. It is generally written as [H3O+] ion and called hydronium ion.
Alkali metal cations also exist as hydrates but the extent of hydration decreases as the size of alkali metal cation increases.
It is not justified to include hydrogen either along with alkali metal of group 1 or halogens of group 17. The position of hydrogen in the periodic table is Anomalous.