Learning Objectives
- Understand the general characteristics of alkali and alkaline earth metals
- Learn about trends in physical and chemical properties
- Study important compounds of sodium, potassium, calcium, and magnesium
- Understand diagonal relationship between Li-Mg and Be-Al
- Learn about biological importance of Na, K, Mg, Ca
Key Concepts
Alkali Metals (Group 1: Li, Na, K, Rb, Cs, Fr)
Electronic configuration: [noble gas] ns¹. Oxidation state: +1. Most reactive metals.
Properties down the group: Atomic radius increases, IE decreases, electronegativity decreases, metallic character increases, reactivity increases.
Physical properties: Soft metals, low melting points, low density (Li, Na, K float on water), silvery white, good conductors.
Chemical properties:
- React vigorously with water: 2Na + 2H₂O → 2NaOH + H₂↑ (exothermic, more vigorous down group).
- React with O₂: Li forms oxide (Li₂O), Na forms peroxide (Na₂O₂), K forms superoxide (KO₂).
- React with halogens, hydrogen (form ionic hydrides).
- Dissolve in liquid ammonia forming blue solution (solvated electrons).
- Flame colours: Li (crimson red), Na (golden yellow), K (violet), Rb (red-violet), Cs (blue).
Important Compounds
NaOH (Caustic soda): Prepared by Castner-Kellner process (electrolysis of brine with Hg cathode). Strong base, deliquescent.
Na₂CO₃ (Washing soda): Solvay process: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ → Na₂CO₃. Decahydrate Na₂CO₃·10H₂O is washing soda.
NaHCO₃ (Baking soda): Mild base, used in fire extinguishers and antacids. 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (on heating).
Alkaline Earth Metals (Group 2: Be, Mg, Ca, Sr, Ba, Ra)
Electronic configuration: [noble gas] ns². Oxidation state: +2. Less reactive than Group 1.
Properties down the group: Similar trends to Group 1 but smaller radii, higher IE, higher melting points.
Chemical properties:
- React with water (Be doesn't, Mg reacts with hot water/steam, Ca and below react with cold water).
- React with O₂ to form oxides. BaO₂ (peroxide).
- Flame colours: Ca (brick red), Sr (crimson), Ba (apple green). Be and Mg have no flame colour.
Important Compounds
CaO (Quicklime): CaCO₃ → CaO + CO₂. Used in cement, glass.
Ca(OH)₂ (Slaked lime): CaO + H₂O → Ca(OH)₂. Used in whitewashing, water treatment.
CaSO₄·½H₂O (Plaster of Paris): 2CaSO₄·2H₂O → 2CaSO₄·½H₂O + 3H₂O (at 373K). Sets by rehydrating to gypsum.
CaCO₃ (Limestone): Used in cement and as flux in metallurgy.
Portland cement: CaO + SiO₂ + Al₂O₃ + Fe₂O₃ mixture.
Diagonal Relationship
Li and Mg: Similar properties due to similar charge/radius ratio. Both form normal oxides, nitrides, organometallic compounds. Carbonates decompose on heating. Hydroxides are weakly basic.
Be and Al: Both amphoteric (oxides/hydroxides dissolve in acid and base). Both form covalent chlorides. Both form carbide that gives methane with water.
Summary
s-Block elements are highly reactive metals with low ionisation energies. Alkali metals (Group 1) are the most reactive, reacting vigorously with water and oxygen. Alkaline earth metals (Group 2) are less reactive but form important compounds. Diagonal relationships (Li-Mg, Be-Al) arise from similar charge/radius ratios. These elements have crucial biological roles.
Important Terms
- Alkali Metals: Group 1 elements with ns¹ configuration
- Alkaline Earth Metals: Group 2 elements with ns² configuration
- Diagonal Relationship: Similarity between element and one diagonally below-right
- Solvay Process: Industrial process for making Na₂CO₃
- Plaster of Paris: CaSO₄·½H₂O, used for casts and moulds
Quick Revision
- Group 1: ns¹, +1 state, most reactive; Group 2: ns², +2 state
- Na + O₂ → Na₂O₂; K + O₂ → KO₂; Li + O₂ → Li₂O
- Flame: Li (red), Na (yellow), K (violet), Ca (red), Ba (green)
- Solvay: NaCl + NH₃ + CO₂ → NaHCO₃ → Na₂CO₃
- Diagonal: Li~Mg, Be~Al
- Biological: Na/K (nerve signals), Ca (bones), Mg (chlorophyll)