CBSE Class 11 Chemistry Notes For Chapter 3 Position Of Hydrogen And Inert Gases In The Periodic Table

CBSE Class 11 Chemistry Notes For Chapter 3 Position Of Hydrogen And Inert Gases In The Periodic Table

The position of hydrogen in the periodic table is controversial. Given its chemical analogy with both the elements of group and that of group- 7A, it can either be placed in group 2A or group 7A. Resemblances of hydrogen with the element.

Arguments in favour of placing hydrogen in group 1A

1. Valency:

The electronic configuration of hydrogen is Is¹ and the general electronic configuration of the elements of group-1A is ns1, i.e., like the elements of group-1A, hydrogen has only one valence electron and its valency is 1.

2. Electropositive character:

Like all group-1A elements, hydrogen tends to form cations by losing one electron.

⇒ \(\mathrm{Na}-e \longrightarrow \mathrm{Na}^{+} ; \mathrm{K}-e \longrightarrow \mathrm{K}^{+} ; \mathrm{H}-e \longrightarrow \mathrm{H}^{+}\)

Like elements of group-1A, hydrogen reacts with electronegative elements such as chlorine, oxygen and sulphur to produce similar types of compounds

Examples: HCl , H₂O , H₂S ; NaCI , Na₂O , Na₂

3. Electrolysis of chloride compounds:

Electrolysis of molten NaCl results in the deposition of metallic sodium at the cathode. Likewise, when an aqueous solution of HCl is electrolysed, H₂ gas is liberated at the cathode.

NaCl ⇌ Na⁺  + Cl^–

Cathode : Na⁺ + e → Na

Anode : Cl^–-e →Cl ; CI + Cl→Cl₂↑,

HCl  ⇌ H⁺ + Cl^–

Cathode : H+ + e →H ; H + H→H₂↑

Anode : Cl^–-e →Cl ; Cl + Cl→Cl₂↑

4. Reducing property:

Like the elements of Gr-IA, hydrogen loses electrons easily and exhibits a reducing property.

5. Formation of alloy:

Hydrogen dissolves in metals like Pd, Pt etc., by adsorption. This occlusion of hydrogen is comparable to the formation of alloys by elements of group IA.

6. Mutual displacement:

Hydrogen atom(s) of hydrochloric, sulphuric or nitric acids can be displaced by the same number of atoms of group-IA elements. Again, atoms of the group-IA elements can be replaced by hydrogen atoms from the salts produced.

7. Formation of stable oxide:

Oxides of group-IA elements are highly stable

Example: Na₂O, K₂O etc.). Similarly, oxide of hydrogen (H20) is also highly stable.

8. Formation of peroxide:

Like the elements of group IA, hydrogen also forms peroxide (H₂O₂). The analogous peroxides of group-IA elements are Na₂O₂, K₂O₂ etc.

9. Electron affinity:

• Hydrogen and the group-IA elements have comparable values of electron affinity.
• In light of the above similarities between the elements of group IA and hydrogen, hydrogen can be placed along with the elements of group 1A.
• However, the placement of hydrogen in group 1A leaves six vacant places in between H and He in the first period.

Arguments in favour of placing hydrogen in group 7A

1. Electronic configuration:

The electronic configuration of hydrogen is 1s¹ and the electronic configuration of the outermost orbit of the elements of group- 7A is ns²np^5- i.e., the outermost orbit of both hydrogen and elements of group-6LA has 1 electron less than the electronic configurations of the nearest inert gas. So, their valency 1.

2. Non-metallic character and atomicity:

Like the dements of group-7A, hydrogen is also non-metal and forms a diatomic molecule.

3. Formation of anion:

Like the elements of group 7A, the hydrogen atom also tends to attain the electronic configuration of Its nearest Inert gas (Me) by accepting an I electron and forming an anion (H^– );

Example:

⇒ \(\mathrm{H}\left(1 s^1\right) \stackrel{+e}{\longrightarrow} \mathrm{H}^{-}\left(1 s^2\right) ; \mathrm{X}\left(n s^2 n p^5\right) \stackrel{+e}{\longrightarrow} \mathrm{X}-\left(n s^2 n p^6\right)\)

Both 7A elements and hydrogen form electrovalent halide and hydride respectively. During the electrolysis of metallic hydrides, like halogens, hydrogen is also liberated at the anode.

NaCl ⇌ Na⁺ + Cl^–

Cathode : Na⁺ + e  → Na

Anode:  Cl^–-e → Cl; Cl+ Cl→ Cl₂ ↑

NaH (molten) ⇌ Na⁺ + H^–

Cathode : Na⁺ + e ιNa

Anode: H^–– e → H; H + H→H₂↑

4. Formation of covalent compounds:

Just like elements of group VIIA, hydrogen reacts with different non-metals to produce covalent compounds with analogous formulas.

Compoundsinvolving H: CH₄, NH₃, H₂O, HF, SiH₄.

Compounds involving Cl: CCl₄, NCl₃, Cl₂O, CIF, SiCl₄

5. Substitution by halogens:

H-atoms of the hydrocarbons can be substituted by Gr-7A elements, partially or completely.

Ionisation potential:

Just like the elements of group 7A, the ionisation, potential of hydrogen is very high but the ionisation potential of alkali metals is quite low. The following table of ionisation potentials shows the comparative picture of ionisation potential quite explicitly.

Maintenance of continuity in the periodic table:

• If H is placed in group 7A, no vacant space remains between H and H. So, continuity in the periodic table is not disturbed.
• From the above discussion, it is apparent that hydrogen is a unique element characterised by peculiar and distinctive unique element characterised by peculiar and distinctive position in the periodic table.
• It is reasonable to set aside a separate position for hydrogen in the periodic table. In the modern periodic table, hydrogen has been J given a completely separate place, at the top of the table.

Position of Inert Gases In The Periodic Table

• Inert gas elements have very stable electronic configurations of their outermost or valence shell (ns² for He and ns²np⁶ for others).
• For this reason, these elements show little or no tendency to lose or gain electrons to form ions to give electrovalent bonds or do not share electrons with other elements to form covalent bonds.
• So the combining capacity or valency of these elements is zero.
• Thus they are placed in group ‘zero’ ofthe periodic table.
• This group forms a bridge between the most electropositive alkali metal elements of group-7A.