Solutions-Class-11-Science-Chemistry-Chapter-8- Elements of Group 1 and 2-Maharashtra Board

(i) Hydrogen shows similarity with alkali metals :

• Hydrogen has electronic configuration 1s¹ which is similar to outer electronic configuration of alkali metals, ns¹.
• Both, hydrogen and Alkali metals form univalent positive ions (+1) by loss of 1 electron.
• Both combine with halogen to form halides.

H₂ + Cl₂ → 2HCI

2Na + Cl₂ → 2NaCl.

(ii) Hydrogen show similarity with halogens :

• Halogens have outer electronic configuration ns²np⁵ which resembles that of hydrogen 1s¹. Both by gaining one electron aquire the electronic configuration of nearest inert gas, ns² (Hydrogen) ns²np⁶ (Halogens).
• Both form univalent negative ion (-1).
• Both react with metals.

2Na(s)+ H₂(g) \(\underrightarrow{Δ}\) 2NaH(s)

2Na + Cl₂ → 2NaC1

• The general outer electronic configuration of alkali metals is ns¹.
• They readily lose one valence shell electron to achieve stable noble gas configuration and hence, they are highly electropositive and are good reducing agents.

Hence, standard reduction potentials of alkali metals have high negative values.

• Electronegativity represents an attractive force exerted by the nucleus on shared electrons.
• Electronegativity of alkaline earth metals decrease down the group. In a group as atomic number increases, atomic radius increases. Nuclear attraction decreases. The tendency to attract shared pair of electron decreases down the group and hence electronegativity decreases.

Sodium dissolves in liquid ammonia forming an ammoniated electron or solvated electron.

Na → Na⁺ + e⁻

Na⁺ + xNH3 → [Na(NH₃)_x]⁺ and

e⁻ + yNH₃ → [e(NH₃)_y] ⁻

Thus, these ions conduct electric current. Hence the solution shows electrical conductivity.

• Beryllium is present in 2nd period. It has a very small atomic size and high ionization enthalpy. Hence it does not form ion but form covalent BeCl₂.
• Magnesium is in 3rd period. Magnesium due to its lower ionization enthalpy and bigger atomic size loses its valence electrons easily forming Mg²⁺

Hence, BeCl₂ is covalent while MgCl₂ is ionic.

All the alkali metals react with water to form corresponding hydroxides with the evolution of hydrogen.

2Na + 2H₂O → 2Na⁺ + 2OH⁻ + H₂ ↑.

Li, Na and K float on water due to release of bubbles of hydrogen gas. The reactions of sodium and potassium are highly exothermic and catch fire.

2HSO₄^- \(\underrightarrow{Electrolysis}\)  H₂S₂Og + 2e^- (at anode).

Hydrolysis of the product, peroxydisulphuric acid is

HO-SO₂-O-O-SO₂-OH + 2H₂O → 2H₂SO₄ + H₂O₂.

2Mg + O₂ → 2MgO

3Mg + N₂ → Mg3N2

BeO + 2HCl_(aq) → BeCl₂ + H₂O

BeO + 2NaOH_(aq) → Na₂BeO₂ + H₂O.

Due to diagonal relationship lithium shows similarities with magnesium.

(i) Both lithium and magnesium form monoxide and nitride when heated in air.

4Li + O₂ \(\underrightarrow{Δ}\) 2Li₂O; 6Li + N₂ \(\underrightarrow{Δ}\) 2Li,N.

2Mg + O₂ \(\underrightarrow{Δ}\) 2MgO; 3Mg + N₂ \(\underrightarrow{Δ}\) Mg₃N₂.

(ii) Lithium carbonate and magnesium carbonate decompose on heating to form their monoxides

Li₂CO₃  Li₂O + CO₂

MgCO3  MgO + CO₂.

(iii) Both, LiCl and MgCl₂ crystallise from aqueous solutions as their hydrates. LiCl.2H₂O and MgCl₂.8H₂O.

Three stages are involved in this industrial process of preparation of hydrogen.

Stage I : Reaction of steam on coke or hydrocarbon in the presence of nickel catalyst, at high temperature (1270 K) yields a mixture of carbon monoxide and hydrogen.

Stage II : Hydrogen is prepared from water gas by mixing it with steam and passing over iron chromate as catalyst (in a shift converter).

Carbon monoxide in the watergas is transformed into carbon dioxide liberating H₂ gas. This is called water gas shift reaction.

Stage III : Carbon dioxide is removed by scrubbing with sodium arsenite solution.

• Water sample containing Ca(HCO₃)₂ and Mg(HCO₃)₂ is hard water and does not give lather with soap.
• When sodium carbonate is added to this water, soluble sodium bicarbonate is formed and insoluble CaCO₃ and MgCO₃ precipitate out. Thus water becomes soft and gives lather with soap.

Ca(HCO₃)₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaHCO₃

Mg(HCO₃)₂(aq) + Na₂CO₃(aq) →  MgCO₃ + 2NaHCO₃

• Hydrogen has three isotopes i.e., hydrogen (), deuterium () and tritium () with mass numbers 1, 2 and 3
• They all contain one proton and one electron but different number of neutrons in the nucleus.

Atomic composition of isotopes of hydrogen:

• Tritium is a radioactive nuclide with a half-life period of 12.4 years and emits low energy ß particles.

Lithium Li

Hydrogen.

Radium

Potassium ion (K⁺).

Lithium aluminium hydride (LiAlH₄)

(c) H-3

(a) Rb

(c) BeCl₂

(b) loses H₂O