CBSE Class 11 Chemistry The S Block Element Notes Set B

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Revision Notes for Class 11 Chemistry Chapter 10 The s-Block Elements

Class 11 Chemistry students should refer to the following concepts and notes for Chapter 10 The s-Block Elements in Class 11. These exam notes for Class 11 Chemistry will be very useful for upcoming class tests and examinations and help you to score good marks

Chapter 10 The s-Block Elements Notes Class 11 Chemistry

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

class_11_chemistry_concept_25

(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 

6. Electropositive or metallic character: All the alkali metals are strongly electropositive or metallic in character

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 

7. Oxidation states: All the alkali metals predominantly exhibit an oxidation state of +1 in their compounds 
Explanation: Alkali metals have only one electron in their valence shell and therefore can lose the single valence electron readily to acquire the stable configuration of a noble gas. Since the second ionization energies are very high, they cannot form divalent ions. Thus, alkali metals are univalent and form ionic compounds 
8. Characteristic flame colouration: All the alkali metals and their salts impart characteristic flame colouration 
Explanation: Alkali metals have very low ionization enthalpies. The energy from the flame of Bunsen burner is sufficient to excite the electrons of alkali metals to higher energy levels. Excited state is quite unstable and therefore when these excited electrons come back to their original energy levels, they emit extra energy, which falls in the visible region of the electromagnetic spectrum and thus appear coloured. Characteristic flame colouration by different alkali metals can be explained on the basis of difference in amount of energy absorbed for excitation of the valence electron 
 
9. Photoelectric effect: Phenomenon of ejection of electrons when electromagnetic radiation of suitable frequency strikes metal surface is called photoelectric effect. Alkali metals exhibit photoelectric effect 
Explanation: Alkali metals have low ionization enthalpies and therefore, the electrons are easily ejected when exposed to light. Among alkali metals,caesium has lowest ionization enthalpy and hence it shows photoelectric effect to the maximum extent 
 
10. Hydration of alkali metal ions: Alkali metal ions are highly hydrated. 
Explanation: Smaller the size of the ion, the greater is the degree of hydration. Thus, Li+ ion gets much more hydrated than Na+ ion which is more hydrated than K+ ion and so on. Therefore, the extent of hydration decreases from Li+ to Cs+. As a result of larger hydration of Li+ ion than Na+ ion, the effective size of hydrated Li+ ion is more than that of hydrated Na+ ion. 
Hydrated ionic radii decrease in the order: Li+ > Na+  > K+  > Rb+ > Cs+  Due to extensive hydration, Li+ ion has lowest mobility in water. 
11. Reducing Nature: Alkali metals are strong reducing agents. This is due to their greater ease to lose electrons 
Explanation: Lithium is the strongest reducing agent. Tendency to act as reducing agent depends on energy requirement involved in three processes  i.e. sublimation, ionization and hydration.

CBSE Class 11 Chemistry Notes - The s-Block Elements 1

 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 

Explanation: In solution the alkali metal atom readily loses the valence electron. Both the cation and the electron combine with ammonia to form ammoniated cation and ammoniated electron. The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution
CBSE Class 11 Chemistry Notes - The s-Block Elements 2 
 
13. Basic strength of hydroxides: Hydroxides of alkali metals are strongly basic and basic strength increases down the group 
Explanation: The Metal—OH bond in the hydroxides of alkali metals is very weak and it can easily ionize to form M+ and OH ions. This accounts for their basic character. Since the ionization energy decreases down the group, the bond between metal and oxygen becomes weak. Therefore, the basic strength of the hydroxides increases accordingly. 
 
(iv) Important Compounds of alkali metals:

CBSE Class 11 Chemistry Notes - The s-Block Elements 3

CBSE Class 11 Chemistry Notes - The s-Block Elements 4


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