S Block Elements Notes
What are S block elements?
The S-block elements are a group of chemical elements that belong to Group 1 (alkali metals) and Group 2 (alkaline earth metals) of the modern periodic table. These elements are called “S-block” because their valence electrons occupy the s orbital of their outermost energy level.
S-block in the Periodic table consists of 14 elements, namely hydrogen (H), lithium (Li), helium (He), sodium (Na), beryllium (Be), potassium (K), magnesium (Mg), rubidium (Rb), calcium (Ca), caesium (Cs), strontium (Sr), francium (Fr), barium (Ba), and radium (Ra).
The alkali metals in Group 1 include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They are highly reactive metals that are characterized by having one valence electron. They are soft, silvery-white metals and are known for their low melting points and densities. They are highly reactive and readily lose their valence electron to form positively charged ions.
The alkaline earth metals in Group 2 include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These metals have two valence electrons and are less reactive than the alkali metals. They are also silvery-white in appearance, but they are denser and have higher melting points compared to alkali metals.
Properties of S-Block Elements
Covalent Character
=> Small Cation and large Anion favour covalency.
=> Order: LiCl > NaCl > KCl > RbCl > CsCl & . LiI > LiBr > LiCl > LiF
=> Greater the charge on the cation, the greater its polarizing power and hence larger is the covalent character: Na+CI- < Mg+2CI2 < AI+3 CI3
=> Greater the charge on the anion, the more easily it gets polarized thereby imparting more covalent character to the compound formed eg. Covalent character increases in order. NaCI < Na2SO4 < Na3PO4
Lattice Energies
Lattice Energy is the amount of energy required to separate one mole of the solid ionic compound into its gaseous ions.
=> Greater the lattice energy, the higher is the melting point of the alkali metals halide and the lower is its solubility in water
Hydration Energy
Hydration energy is defined as the amount of energy released when one mole of gaseous ions combine with water to form hydrated ions.
M+ (g) + aq → M+ (aq) + hydration energy
X- (g) + aq → X- (aq) + hydration energy
=> Higher the hydration energy of the ions greater the solubility of the compound in water.
=> The solubility of most alkali metal halides except those of fluorides decreases on descending the group since the decrease in hydration energy is more than the corresponding decrease in the lattice energy.
=> Due to the high hydration energy of Li+ ion, Lithium halides are soluble in water except for LiF which is sparingly soluble due to its high lattice energy.
=> For the same alkali metal, the melting point decreases in the order
fluoride > chloride > bromide > iodide
=> For the same halide ion, the melting point of lithium halides is lower than those of the corresponding sodium halides and thereafter they decrease as we move down the group from Na to Cs.
=> The low melting point of LiCl (887 K) as compared to NaCl is probably because LiCl is covalent in nature and NaCl is ionic.
Anomalous Behavior of Lithium and diagonal relationship with Magnesium
=> Li has anomalous properties due to i) Very small size ii) High polarizing Power
=> Lithium shows a diagonal relationship with magnesium because both elements have almost the same polarizing power.
=> The melting point and boiling point of lithium are comparatively high.
=> Lithium is much harder than other alkali metals. Magnesium is also hard metal.
=> Lithium reacts with oxygen least readily to form normal oxide whereas other alkali metals form peroxides and superoxides.
=> LiOH like Mg (OH)2 is a weak base. Hydroxides of other alkali metals are strong bases.
=> Due to their appreciable covalent nature, the halides and alkyls of lithium and magnesium are soluble in organic solvents.
=> Unlike elements of group 1 but like magnesium. Lithium forms nitride with nitrogen.6Li + N2 → 2Li3N
=> LiCl is deliquescent and crystallizes as a hydrate, LiCI2H2O. Other alkali metals do not form hydrates. also forms hydrate, MgCI2.8H2O
=>, Unlike other alkali metals, lithium reacts directly with carbon to form an ionic carbide. Magnesium also forms a similar carbide.
=> The carbonates, hydroxides and nitrates of lithium as well as magnesium decompose on heating.
Li2CO3 → Li2O + CO2
MgCO3 → MgO + CO2
2LiOH → Li2O + H2O
Mg (OH)2 → MgO + H2O
4LiNO3 → 2Li2O + 4NO2 + O2
2Mg ( NO3)2 → 2Mg + 4NO2 +O2
=> The corresponding salts of other alkali metals are stable towards heat.
Lithium nitrate, on heating, decomposes to give lithium oxide, Li2O whereas other alkali metals nitrate decomposes to give the corresponding nitrite.
4LiNO3 → 2Li2O + 4NO2 + O2
2NaNO3 → 2NaNO2 + O2
2KNO3 → 2KNO2 + O2
=> Li2CO3, LiOH, LiF and Li3PO4 are the only alkali metal salts which are insoluble in water. The corresponding magnesium compounds are also insoluble in water.
=> Hydrogen carbonates of both lithium and magnesium can not be isolated in solid state. Hydrogen carbonates of other alkali metals can be isolated in the solid state.
Sodium Hydroxide (NaOH):
=> NaOH is stable towards heat but is reduced to metal when heated with carbon
2NaOH + 2C → 2Na +2CO + H2
FeCl3 + 3NaOH →Fe(OH)3 + 3NaCl
NH4Cl + NaOH → NaCl + NH3 (pungent smell) + H2O