Group 1: Alkali Metals
Li, Na, K, Rb, Cs, Fr.
General Characteristics
- General electronic configuration:
ns^1. - Highly reactive metals.
- Low ionisation enthalpy.
- Large atomic size.
- +1 oxidation state.
- Soft metals, can be cut with a knife.
- Low melting and boiling points.
Trends in Properties
| Property | Trend Down the Group |
|---|---|
| Atomic radius | Increases |
| Ionisation enthalpy | Decreases |
| Melting point | Decreases |
| Density | Increases (generally) |
| Reactivity | Increases |
| Electropositive character | Increases |
Chemical Properties
Reaction with water: 2Na + 2H2O -> 2NaOH + H2 (vigorous, exothermic).
Reaction with oxygen:
- Li forms Li2O (monoxide).
- Na forms Na2O2 (peroxide).
- K, Rb, Cs form MO2 (superoxides).
Reaction with halogens: 2Na + Cl2 -> 2NaCl.
Reaction with hydrogen: 2Na + H2 -> 2NaH (ionic hydrides).
Anomalous Behaviour of Lithium
- More covalent character (small size, high polarising power).
- Forms Li2O (not peroxide/superoxide).
- Reacts with N2 to form Li3N.
- LiOH is weaker base than other alkali hydroxides.
- Lithium resembles Mg (diagonal relationship).
Important Compounds
NaOH (Caustic Soda):
- Manufacture: Castner-Kellner process (electrolysis of NaCl).
- Properties: Strong base, deliquescent, corrosive.
- Uses: Soap, paper, textile, petroleum refining.
Na2CO3 (Washing Soda):
- Manufacture: Solvay process.
- Properties: White crystalline, alkaline in solution.
- Uses: Glass, soap, water softening.
Flame Test
Alkali metals give characteristic colours:
- Li: Crimson red.
- Na: Golden yellow.
- K: Pale violet (lilac).
- Rb: Red-violet.
- Cs: Blue.
Group 2: Alkaline Earth Metals
Be, Mg, Ca, Sr, Ba, Ra.
General Characteristics
- General electronic configuration:
ns^2. - +2 oxidation state.
- Harder than alkali metals.
- Higher melting points.
- Less reactive than Group 1.
Trends
| Property | Trend Down the Group |
|---|---|
| Atomic radius | Increases |
| Ionisation enthalpy | Decreases |
| Melting point | Decreases |
| Density | Increases |
| Reactivity | Increases |
Chemical Properties
Reaction with water: Ca + 2H2O -> Ca(OH)2 + H2 (less vigorous than Na).
Mg reacts with hot water/steam only.
Reaction with oxygen: 2Ca + O2 -> 2CaO.
Reaction with halogens: Ca + Cl2 -> CaCl2.
Reaction with acids: Mg + 2HCl -> MgCl2 + H2.
Anomalous Behaviour of Beryllium
- Forms covalent compounds (small size, high polarising power).
- Be(OH)2 is amphoteric (others are basic).
- Be does not react with water.
- Be forms complexes.
- Diagonal relationship with Al.
Important Compounds
CaO (Quicklime):
- Manufacture: Thermal decomposition of CaCO3:
CaCO3 -> CaO + CO2. - Uses: Cement, steel, water treatment.
CaCO3 (Limestone):
- Naturally abundant.
- Thermal decomposition gives CaO.
- Reacts with acids:
CaCO3 + 2HCl -> CaCl2 + H2O + CO2. - Used in cement, glass, building materials.
Ca(OH)2 (Slaked Lime):
- Manufacture:
CaO + H2O -> Ca(OH)2. - Uses: Whitewash, mortar, water softening.
Cement: Manufactured from limestone and clay. Contains CaO, SiO2, Al2O3, Fe2O3.
Biological Importance
Sodium and Potassium
- Na+ maintains osmotic balance and fluid volume.
- K+ maintains resting membrane potential in nerve cells.
- Na+/K+ ATPase pump maintains ion gradients.
- Important for muscle contraction and nerve impulse transmission.
Calcium and Magnesium
- Ca^2+ essential for bone and teeth formation.
- Ca^2+ required for blood clotting, muscle contraction.
- Mg^2+ central atom in chlorophyll (photosynthesis).
- Mg^2+ activates many enzymes.
Worked Examples
Example 1: Write reactions when sodium is exposed to air.
Solution: Fresh surface tarnishes rapidly: 4Na + O2 -> 2Na2O. Further: Na2O + H2O -> 2NaOH. With CO2: 2NaOH + CO2 -> Na2CO3 + H2O.
Example 2: Why is Na stored under kerosene? Solution: Na reacts violently with air (O2, moisture) and water. Kerosene prevents contact.
Common Mistakes
- Storing Li differently: Li is stored in paraffin wax (lighter than kerosene and floats).
- Peroxide vs superoxide: Na forms peroxide; K, Rb, Cs form superoxides.
- Group 2 reactivity with water: Mg reacts slowly with hot water, not cold.
- Be is unique: Be(OH)2 is amphoteric; Mg(OH)2 to Ba(OH)2 are basic.
ISC Exam Focus
- Theory (70%): Group trends, reactions, anomalous behaviour, important compounds.
- Application (30%): Equations of reactions, uses of compounds, biological importance.
- ISC frequently asks: "Explain the variation in ... down Group 1/2" and "Why is Li anomalous?"
- Comparative questions on alkali vs alkaline earth metals.
Self-Test Questions
Q1: Write the general electronic configuration of Group 1 and Group 2 elements. Answer: Group 1: ns^1. Group 2: ns^2.
Q2: Why are alkali metals stored under kerosene? Answer: They react vigorously with air (O2) and moisture. Kerosene provides an inert environment.
Q3: What is the product when Na reacts with excess O2?
Answer: 2Na + O2 -> Na2O2 (sodium peroxide). Li gives Li2O.
Q4: Write Solvay process equations for Na2CO3 manufacture.
Answer: NaCl + NH3 + CO2 + H2O -> NaHCO3 + NH4Cl. 2NaHCO3 -> Na2CO3 + H2O + CO2.
Q5: State the biological importance of Ca and Mg. Answer: Ca: bones, teeth, blood clotting, muscle contraction. Mg: chlorophyll, enzyme activation.
Q6: Explain the diagonal relationship between Li and Mg. Answer: Both form nitrides (Li3N, Mg3N2), carbonates decompose on heating, hydroxides are weaker bases, and some compounds have covalent character.
