Position of Hydrogen

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.

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

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.