Unit-10 The S-Block elements

Updated: Jan 16

Important points :

1) S-block consists of group-1 (alkali metals) and group 2(alkaline earth metals)

2) GROUP-1

_ Elements-I (LiNa Ki Ruby Cse Friendship hai)

_Name -Alkali because hydroxides of these elements are basic/alkaline in nature

_Most abundant: sodium; potassium

_less abundant: lithium; rubidium; caesium

_highly radioactive; Francium

_223 Fr (the longest-lived isotope of francium) has a half-life of only 21 minutes

_general electronic configuration: ns1

3) GROUP-2

_Elements- Be, Mg, Ca, Sr, Ba, Ra (Beta Mange Car Scooter Baap Razi)

_Name- Alkaline Earth metals because their oxides and hydroxides are alkaline in nature

And these metal oxides are found in the earth’s crust.

_Exception= Berillium

_ Radium is rarest element.

 Atomic radius : Atomic radius of alkali metals are greater than alkaline

earth metals.

 Hydration enthalpy : Decreases with increases in ionic sizes.

 Ionic mobility : Smaller the size of ion, more highly it is hydrated and

hence lower is its ionic mobility.

Li+ < Na+ < K+ < Rb+ < Cs+

 Ionisation enthalpies : 1st I.E. of group 1st is smaller than group 2nd

elements but 2nd I.E. of group 2nd is smaller than group 1st elements.

 Flame colouration : Due to low I.E., s-block elements and their salts

imparts the characteristics colour of oxidising flame (except Be and Mg). Be

and Mg do not show flame colouration because they have a small size and

very high ionisation enthalpy.

 Reducing character : Due to large negative electrode potentials alkali

metals are a stronger reducing agent than alkaline earth metals.

 Reactivity towards air :

4Li + O2 —→ 2Li2O (Lithium oxide)

2Na + O2 —→ Na2O2 (Sodium peroxide)

M + O2 —→ MO2 (M = K, Rb, Cs metal superoxide)

Alkaline earth metals being smaller in size do not form superoxides.

 Reactivity towards H2O :

2M + 2H2O —→ 2MOH + H2

(Alkali metal)

M + 2H2O —→ M(OH)2 + H2

(Alkaline earth metals)

 Reactivity towards hydrogen :

2M + H2 —→ 2MH (M = Li, Na, K, Rb, Cs)

M + H2 —→ MH2 (M = Mg, Ca, Sr, Ba)

2BeCl2 + LiAlH4 —→ 2BeH2 + LiCl + AlCl3.

 Reactivity towards halogens :

2M + X2 —→ 2MX (M = Li, Na, K, Rb, Cs)

M + X2 —→ MX2 (M = Mg, Ca, Sr, Ba)

BeO + C + Cl2 BeCl2 + CO

 Solution in liquid ammonia : The fresh solution of alkali metals and

alkaline earth metals (except Be and Mg) is deep blue, paramagnetic and

highly reducing due to presence of ammoniated electrons.

 Solubility of alkaline earth metal carbonate in water :

Li2CO3 < Na2CO3 < K2CO3 < RbCO3 < Cs2CO3

 Solubility of alkaline earth metal carbonates in water.

BaCO3 < SrCO3 < CaCO3 < MgCO3 < BeCO3

 Solubility of alkaline earth metal sulphates in water :

BaSO4 < SrSO4 < CaSO4 < MgSO4 < BeSO4

 Thermal stability of alkali metal carbonates :

Li2CO3 < Na2CO3 < K2CO3 < Rb2CO3 < Cs2CO3

 Thermal stability of alkaline earth metal carbonates :

BeCO3 < MgCO3 < CaCO3 < SrCO3 < BaCO3

 Anomalous behaviour of Li and Be : It is due to very small size, high

I.E. and high polarising power (i.e., charge/radius)

 Diagonal relationship (similarities) between Li and Mg :

(I) Both Li and Mg are hard

(II) Both react with N2 to form nitrides.

6Li + N2 —→ 2Li3N

3Mg + N2 —→ Mg3N2

(iii) Decomposition of carbonates :

Li2CO3 Li2O + CO2

MgCO3 MgO + CO2

(iv) Both LiCl and MgCl2 are deliquescent. They form hydrates salts

LiCl.2H2O and MgCl2.6H2O.

(v) Decomposition of nitrates :

4LiNO3 2Li2O + 4NO2 + O2

2Mg(NO3)2 2MgO + 4NO2 + O2

 Diagonal relationship (similarities) between Be and Al :

(i) Both are passive to acids due to formation of oxide layer.

(ii) Hydroxides of both dissolve in alkali to form [Be(OH)4]2– and


(iii) Chloride of both has bridged structure.

(iv) Both have tendency to form complexes of BeF42–, AlF63–.

 Manufacturing of washing soda (Na2CO3.10H2O) :

Solvay process :

NH3(g) + CO2(g) + H2O (l) —→ NH4HCO3 (aq)

NH4HCO3 (aq) + NaCl (aq) —→ NaHCO3(s) + NH4Cl (aq)

2NaHCO3 Na2CO3 + H2O(l) + CO2(g)

2NH4Cl (aq) + Ca(OH)2 → CaCl2(s) + 2H2O(l) + 2NH3 (g)

 Manufacturing of caustic soda (NaOH) : Castner-Kellner cell.

Cathode : Na+ + e– Na-Hg

Anode : Cl– —→ + e–

2Na-Hg + 2H2O —→ 2NaOH + 2Hg + H2

 Plaster of paris : (CaSO4.½H2O)

2(CaSO4.2H2O) 2(CaSO4).H2O + 3H2O

[MJ1]Half-life, in radioactivity, the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay,

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