The S-block Elements

This is a long answer type question as classified in NCERT Exemplar

(i) The alkali metals dissolve in liquid ammonia and give a blue solution, which is conductive in nature. A solution of sodium in liquid ammonia at -30C conducts electricity. The ammoniated electrons are responsible for the blue color of the solution as they absorb energy in the visible region of light and impart blue color to the solution. Both the ammoniated cations and ammoniated electrons are responsible for the electrical conductivity of the solution.

Na + (x+y)NH₃→ [Na (NH₃)_x]⁺ + [e (NH₃)_y]⁻

(ii) The blue color changes to bronze color in concentrated solution due to the formation of a cluster of metal ions. The standing blue solution liberates hydrogen gas with the formation of amide.

M⁺ + e^? + NH₃? MNH₂ + 12H₂

This is a long answer type question as classified in NCERT Exemplar

(i) Alkaline earth metal compounds are less ionic than alkali metals because of small size and more effective nuclear charge.

(ii) Oxides of alkali metals are more basic than alkaline earth metals. This are water soluble and highly exothermic. The hydroxides of alkaline earth metals are less basic than alkali metals.

(iii) Alkaline earth metals give oxosalts. The reactivity of alkali metals is faster than the reactivity of alkaline earth metal. The reactivity of alkaline earth metal is less due to small size and more effective nuclear charge.

(iv) The oxo salts of alkali metals are less soluble than the oxo salts of alkaline earth metal because of small size of cation and high hydration enthalpy.

(v) The thermal stability of the oxo salts of alkali metals are more than the alkaline earth metals. The sodium carbonate is stable towards heat.

(a) Alkali metals have low ionization enthalpies.

They have a strong tendency to lose 1 electron to form unipositive ions. Thus they show an oxidation state of +1 and are strongly electropositive.

(b) Valence electrons of alkali metals like Na and K easily absorb energy from the
flame and are excited to higher energy levels. When these electrons return to the ground state, the energy is emitted in the form of light.
Magnesium atom has small size so electrons are strongly bound to the nucleus. [ Thus they need large amount of energy for excitation of electrons to higher
energy levels which is not possible in Bunsen flame.

(c)Due to the small size of Li+ it has a strong positive field which attracts the negative charge so strongly that it does not permit the oxide ion, 02- to combine with another oxygen atom to form peroxide ion.
(d)Since, among alkali metals, lithium has the most negative electrode potential (E° = -3.04 V) so, it is the strongest reducing agent in the aqueous solution.