CBSE Class 11 Chemistry The S Block Element Notes Set B

Class: XI

Chapter: The s-Block Elements

Top concepts

1. The s-block elements of the periodic table are those in which the last electron enters the outermost s-orbital
2. Elements of group1 are called alkali metals and elements of group 2 are called alkaline earth metal

3. Group1 elements: Alkali metals

(i) Group 1 elements are called alkali metals because they form hydroxides on reaction with water which are strongly alkaline in nature

(ii) The general electronic for alkali metals is [noble gas] ns¹

(iii)Trends in properties of alkali metals:

1. Atomic and ionic radii: Alkali metals have the largest atomic and ionic radii in their respective periods of the periodic table. On moving down the group, the atomic and ionic radii increase

Explanation: As we move in a period, the atomic radius and ionic radius tend to decrease due to increase in the effective nuclear charge. Therefore alkali
metals have largest atomic and ionic radii in their respective group. On moving down the group there is increase in the number of shells .Thus there
is an increase in distance between nucleus and outermost electrons which eventually increases atomic and ionic radii

2. Ionisation enthalpies: Alkali metals have the lowest ionization enthalpy in each period. Within the group, the ionization enthalpies of alkali metals decrease down the group

Explanation:Atoms of alkali metals are largest in their respective periods and therefore, the valence electrons are loosely held by the nucleus. By losing the valence electron, they acquire stable noble gas configuration. This accounts for their ease to lose electrons and hence they, have low ionization enthalpies

3) The second ionization enthalpies of alkali metals are very high.

Explanation: When an electron is removed from the alkali metals, they form monovalent cations which have very stable electronic configurations (same as that of noble gases).Therefore it becomes difficult to remove the second electron from the stable noble gas configurations and hence their second ionization enthalpy values (IE2) are very high

4. Melting and boiling points: Alkali metals are soft and have low melting and boiling points Explanation: Alkali metals have only one valence electron per metal atom and therefore, the energy binding the atoms in the crystal lattice of the metal is low. Thus, the metallic bonds in these metals are not very strong and consequently, their melting and boiling points decrease on moving down from Li to Cs

5. Density: Densities of alkali metals are quite low as compared to other metals. The densities increase on moving down the group. But K is lighter
than Na
Explanation: The densities of metallic elements depend upon the type of packing of atoms in metallic state and also on their size. The alkali metals have close packing of metal atoms in their lattice and because of the large size of their atoms, they have low densities. On moving down the group from Li to Cs, there is increase in atomic size as well as atomic mass. But the increase in atomic mass is more and compensates the increase in atomic size. As a result, the densities (mass/volume) of alkali metals gradually increase from Li to Cs. K is lighter than Na due to increase in atomic size of K 

Explanation: The electropositive character of an element is expressed in terms of the tendency of its atom to release electrons:

M → M ⁺e ^-

As alkali metals have low ionization enthalpies, their atoms readily lose their valence electron. These elements are, therefore, said to have strong electropositive or metallic character. Since, the ionization energies decrease down the group, the electron releasing tendency or electropositive character is expected to increase down the group 

 Lithium being small in size has high ionization enthalpy. On the other hand because of small size it is extensively hydrated and has very high hydration enthalpy. This high hydration enthalpy compensates the high energy needed to remove electron (in second step). Thus Li has greater tendency to lose electrons in solution than other alkali metals. Thus Li is the strongest reducing agent.

12. Solutions in liquid ammonia: Alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature 

 

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