JEE Mains S-Block Elements

Cheatsheet Content

1. Alkali Metals (Group 1) 1.1 General Characteristics Electronic Configuration: $[Noble \ Gas] \ ns^1$ Oxidation State: Always $+1$ Atomic & Ionic Radii: Increase down the group. Ionization Enthalpy: Low, decreases down the group. Hydration Enthalpy: Decreases down the group ($Li^+ > Na^+ > K^+ > Rb^+ > Cs^+$). Flame Coloration: Li: Crimson Red Na: Golden Yellow K: Lilac (Pale Violet) Rb: Red-Violet Cs: Blue 1.2 Chemical Properties Reactivity: Highly reactive, increases down the group. Reaction with Air: Li: Forms $Li_2O$ (Lithium oxide) Na: Forms $Na_2O_2$ (Sodium peroxide) K, Rb, Cs: Form $MO_2$ (Superoxides) Reaction with Water: $2M(s) + 2H_2O(l) \rightarrow 2M^+(aq) + 2OH^-(aq) + H_2(g)$. Highly exothermic. Reaction with Hydrogen: Form ionic hydrides, $MH$. Reaction with Halogens: Form ionic halides, $MX$. Reducing Nature: Strong reducing agents. $Li$ is the strongest in aqueous solution due to high hydration enthalpy. Solutions in Liquid Ammonia: Deep blue solutions, conducting, paramagnetic. On standing, liberate $H_2$ and form amides ($MNH_2$). 1.3 Important Compounds of Sodium Sodium Carbonate (Washing Soda), $Na_2CO_3 \cdot 10H_2O$: Preparation: Solvay process ($NH_3 + H_2O + CO_2 \rightarrow NH_4HCO_3$; $NaCl + NH_4HCO_3 \rightarrow NaHCO_3 \downarrow + NH_4Cl$; $2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$). Properties: White crystalline solid, soluble in water, alkaline solution due to hydrolysis. Sodium Chloride (Common Salt), $NaCl$: Sources: Seawater, rock salt. Uses: Food preservative, raw material for $NaOH, Na_2CO_3$. Sodium Hydroxide (Caustic Soda), $NaOH$: Preparation: Castner-Kellner cell (electrolysis of brine). Properties: White transluscent solid, deliquescent, strongly alkaline. Sodium Bicarbonate (Baking Soda), $NaHCO_3$: Preparation: Solvay process (intermediate product). Properties: Less soluble than $Na_2CO_3$, mild antiseptic, antacid. 2. Alkaline Earth Metals (Group 2) 2.1 General Characteristics Electronic Configuration: $[Noble \ Gas] \ ns^2$ Oxidation State: Always $+2$ Atomic & Ionic Radii: Increase down the group (smaller than Group 1). Ionization Enthalpy: Low, decreases down the group (higher than Group 1). Hydration Enthalpy: Decreases down the group ($Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+}$). Flame Coloration: Be, Mg: No coloration (due to high ionization energy, electrons not easily excited). Ca: Brick Red Sr: Crimson Ba: Apple Green 2.2 Chemical Properties Reactivity: Less reactive than alkali metals, increases down the group. Reaction with Air: Be, Mg: Form oxide and nitride. Ca, Sr, Ba: Form oxide. Reaction with Water: $M(s) + 2H_2O(l) \rightarrow M(OH)_2(aq) + H_2(g)$. Reactivity increases down the group. Reaction with Halogens: Form halides, $MX_2$. Reducing Nature: Strong reducing agents, but weaker than alkali metals. Solutions in Liquid Ammonia: Deep blue-black solutions, conducting. 2.3 Important Compounds Calcium Oxide (Quicklime), $CaO$: Preparation: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$ (calcination). Properties: Basic oxide, reacts with water to form $Ca(OH)_2$ (slaking of lime). Calcium Hydroxide (Slaked Lime), $Ca(OH)_2$: Properties: White amorphous powder, sparingly soluble in water (milk of lime). Clear solution is lime water. Uses: Building material, whitewash, preparation of bleaching powder. Calcium Carbonate (Limestone), $CaCO_3$: Forms: Limestone, marble, chalk. Uses: Building material, antacid, raw material for $CaO$. Calcium Sulphate (Gypsum), $CaSO_4 \cdot 2H_2O$: Plaster of Paris (POP): $2(CaSO_4 \cdot 2H_2O) \xrightarrow{393K} (CaSO_4)_2 \cdot H_2O + 3H_2O$. Properties of POP: Sets with water, used in casts, statues. Magnesium Hydroxide, $Mg(OH)_2$: Properties: Sparingly soluble, 'milk of magnesia' is an antacid. 3. Anomalous Behavior of Lithium and Beryllium 3.1 Lithium vs. Other Alkali Metals Smallest size, highest polarizing power. Forms covalent compounds (e.g., $LiCl$ is partially covalent). Hardest metal, highest melting point. Reacts slowly with water. Forms only oxide ($Li_2O$), not peroxide or superoxide. Forms nitride ($Li_3N$). Diagonal relationship with Magnesium (similar size, polarizing power). 3.2 Beryllium vs. Other Alkaline Earth Metals Smallest size, highest polarizing power. Forms covalent compounds. Higher melting and boiling points. Does not react with water even at high temperatures. Forms stable carbides. Diagonal relationship with Aluminum. 4. Diagonal Relationships Lithium (Group 1) and Magnesium (Group 2): Similar electronegativity and polarizing power. Form nitrides ($Li_3N, Mg_3N_2$). Hydroxides are weak bases. Carbonates decompose on heating. Form complex compounds. Beryllium (Group 2) and Aluminum (Group 13): Similar electronegativity and charge/radius ratio. Form covalent compounds. Amphoteric oxides and hydroxides ($BeO, Al_2O_3$; $Be(OH)_2, Al(OH)_3$). Carbides react with water to give methane. Resist attack by acids due to oxide film. 5. General Trends and Reactions 5.1 Hydrides Alkali metals form ionic hydrides ($MH$). Alkaline earth metals form ionic hydrides ($MH_2$), except $BeH_2$ which is polymeric and covalent. 5.2 Halides Alkali metal halides ($MX$) are ionic, except $LiX$ which shows some covalent character. Alkaline earth metal halides ($MX_2$) are generally ionic, except $BeCl_2$ which is covalent and polymeric. 5.3 Solubility of Compounds Hydroxides: Solubility increases down both groups. ($LiOH Sulphates: Solubility decreases down Group 2 ($BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$). Carbonates: Solubility decreases down Group 2 ($MgCO_3 > CaCO_3 > SrCO_3 > BaCO_3$).