Chemistry Class 11 NCERT Solutions: Chapter 10 the s Block Elements Part 2

Q: 2. Discuss the general characteristics and gradation in properties of alkaline earth metals.

Answer:

General characteristics of alkaline earth metals are as follows.

(i) The general electronic configuration of alkaline earth metals is [noble gas] ns2.

(ii) These metals lose two electrons to acquire the nearest noble gas configuration. Therefore, their oxidation state is +2.

(iii)These metals have atomic and ionic radii smaller than that of alkali metals. Also, when moved down the group, the effective nuclear charge decreases and this causes an increase in their atomic radii and ionic radii.

(iv)Since the alkaline earth metals have large size, their ionization enthalpies are found to be fairly low. However, their first ionization enthalpies are higher than the corresponding group 1 metals.

(v) These metals are lustrous and silvery white in appearance. They are relatively less soft as compared to alkali metals.

(vi)Atoms of alkaline earth metals are smaller than that of alkali metals. Also, they have two valence electrons forming stronger metallic bonds. These two factors cause alkaline earth metals to have high melting and boiling points as compared to alkali metals.

(vii) They are highly electropositive in nature. This is due to their low ionization enthalpies. Also, the electropositive character increases on moving down the group from Be to Ba.

(viii) Ca, Sr, and Ba impart characteristic colours to flames.

Ca - Brick red

Sr - Crimson red

Ba - Apple green

In Equation and Equation , the electrons are too strongly bound to be excited. Hence, these do not impart any colour to the flame. The alkaline earth metals are less reactive than alkali metals and their reactivity increases on moving down the group. Chemical properties of alkaline earth metals are as follows.

(i) Reaction with air and water: Equation are almost inert to air and water because of the formation of oxide layer on their surface.

(a) Powdered Equation burns in air to form Equation and Equation .

(b) Equation , being more electropositive, burns in air with a dazzling sparkle to form Equation and Equation .

(c) Equation and Equation react readily with air to form respective oxides and nitrides.

(d) Equation react vigorously even with cold water.

(ii) Alkaline earth metals react with halogens at high temperatures to form halides.

Equation

(iii) All the alkaline earth metals, except Be, react with hydrogen to form hydrides.

(iv) They react readily with acids to form salts and liberate hydrogen gas.

Equation

(v) They are strong reducing agents. However, their reducing power is less than that of alkali metals. As we move down the group, the reducing power increases.

(Vi) Similar to alkali metals, the alkaline earth metals also dissolve in liquid ammonia to give deep blue coloured solutions.

Equation

Q: 3. Why are alkali metals not found in nature?

Answer:

Alkali metals include lithium, sodium, potassium, rubidium, cesium, and francium. These metals have only one electron in their valence shell, which they lose easily, owing to their low ionization energies. Therefore, alkali metals are highly reactive and are not found in nature in their elemental state.

Q: 4. Find the oxidation state of sodium in Equation .

Answer:

Let the oxidation state of Equation be Equation . The oxidation state of oxygen, in case of peroxides, is Equation

Therefore,

Equation

Equation

Equation

Equation

Therefore, the oxidation sate of sodium is Equation

Q: 5. Explain why is sodium less reactive than potassium?

Answer:

In alkali metals, on moving down the group, the atomic size increases and the effective nuclear charge decreases. Because of these factors, the outermost electron in potassium can be lost easily as compared to sodium. Hence, potassium is more reactive than sodium.

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