s-block Elements: Groups 1 & 2

The Hook: Why Does Sodium Explode in Water?

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The Core Concept: s-block Elements

Definition

s-block elements are those in which the last electron enters the s orbital.

$$\boxed{\text{Valence configuration: } ns^{1-2}}$$

Two Groups

Group	Name	Valence Config	Elements
Group 1	Alkali Metals	ns¹	Li, Na, K, Rb, Cs, Fr
Group 2	Alkaline Earth Metals	ns²	Be, Mg, Ca, Sr, Ba, Ra

In simple terms: These elements have their outermost electrons in s orbitals. Group 1 has 1 electron, Group 2 has 2 electrons. They LOVE to lose these electrons to achieve noble gas configuration!

Interactive Demo: Explore s-Block Elements

See the properties and trends of alkali and alkaline earth metals on the periodic table.

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Group 1: Alkali Metals (The Explosive Squad)

Electronic Configuration

Element	Z	Configuration	Name Origin
Li	3	[He] 2s¹	Greek: lithos (stone)
Na	11	[Ne] 3s¹	Latin: natrium
K	19	[Ar] 4s¹	Latin: kalium
Rb	37	[Kr] 5s¹	Latin: rubidius (red)
Cs	55	[Xe] 6s¹	Latin: caesius (blue)
Fr	87	[Rn] 7s¹	France (radioactive)

Physical Properties Trends

Down the Group (Li → Cs)

Property	Trend	Reason
Atomic radius	Increases →	New shell added
Ionic radius	Increases →	Same reason
Density	Irregular (K < Na)	Volume increases faster than mass
Melting point	Decreases →	Weaker metallic bonding
Ionization energy	Decreases →	Electron farther from nucleus
Electronegativity	Decreases →	Lower attraction for electrons

Chemical Properties

1. Reaction with Water

$$\boxed{2M + 2H_2O \rightarrow 2MOH + H_2 \uparrow}$$

Reactivity: Li < Na < K < Rb < Cs

Observations:

Metal	Reaction
Li	Gentle fizzing, floats, moves around
Na	Vigorous, melts into ball, may ignite
K	Violent, lilac flame, catches fire immediately
Rb, Cs	EXPLOSIVE, instant ignition

Why increasing reactivity?

• Larger size → lower ionization energy
• Easier to lose electron → faster reaction
• More heat released → more violent

2. Reaction with Oxygen

Different products based on metal!

$$\boxed{4Li + O_2 \rightarrow 2Li_2O \text{ (normal oxide)}}$$ $$\boxed{2Na + O_2 \rightarrow Na_2O_2 \text{ (peroxide)}}$$ $$\boxed{K + O_2 \rightarrow KO_2 \text{ (superoxide)}}$$

Metal	Product	Formula	O.S. of O
Li	Oxide	Li₂O	-2
Na	Peroxide	Na₂O₂	-1
K, Rb, Cs	Superoxide	MO₂	-½

Why different products? Larger cations can stabilize larger anions (O₂²⁻, O₂⁻)

3. Reaction with Halogens

$$\boxed{2M + X_2 \rightarrow 2MX}$$

• Forms ionic halides (MF, MCl, MBr, MI)
• Highly exothermic reactions
• Reactivity: F₂ > Cl₂ > Br₂ > I₂

4. Reaction with Hydrogen

$$\boxed{2M + H_2 \xrightarrow{\Delta} 2MH}$$

• Forms ionic hydrides (LiH, NaH, etc.)
• H has -1 oxidation state (hydride ion: H⁻)
• Strong reducing agents

Important Compounds

Sodium Hydroxide (NaOH) - Caustic Soda

Preparation:

$$\boxed{2NaCl + 2H_2O \xrightarrow{\text{electrolysis}} 2NaOH + Cl_2 + H_2}$$

Properties:

• White, deliquescent solid
• Highly soluble in water (exothermic)
• Strong base (pH ≈ 14)

Uses:

• Soap and detergent manufacturing
• Paper industry
• Petroleum refining

Sodium Carbonate (Na₂CO₃) - Washing Soda

Formula: Na₂CO₃·10H₂O (decahydrate)

Solvay Process:

$$NaCl + NH_3 + CO_2 + H_2O \rightarrow NaHCO_3 + NH_4Cl$$ $$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$$

Uses:

• Water softening
• Glass manufacturing
• Cleaning agent

Sodium Hydrogen Carbonate (NaHCO₃) - Baking Soda

Properties:

• Weakly basic
• Decomposes on heating:

$$\boxed{2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2 \uparrow}$$

Uses:

• Baking (CO₂ makes cakes fluffy)
• Fire extinguishers
• Antacid

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Group 2: Alkaline Earth Metals (The Moderate Squad)

Electronic Configuration

Element	Z	Configuration	Name
Be	4	[He] 2s²	Beryllium
Mg	12	[Ne] 3s²	Magnesium
Ca	20	[Ar] 4s²	Calcium
Sr	38	[Kr] 5s²	Strontium
Ba	56	[Xe] 6s²	Barium
Ra	88	[Rn] 7s²	Radium (radioactive)

Physical Properties Trends

Property	Trend (Be → Ba)	Value Range
Atomic radius	Increases	112 pm (Be) → 215 pm (Ba)
Ionic radius	Increases	31 pm (Be²⁺) → 135 pm (Ba²⁺)
Ionization energy	Decreases	899 (Be) → 503 (Ba) kJ/mol
Electronegativity	Decreases	1.5 (Be) → 0.9 (Ba)
Metallic character	Increases	Be (least) → Ba (most)

Chemical Properties

1. Reaction with Water

Reactivity: Be < Mg < Ca < Sr < Ba

Metal	Reaction with Water
Be	No reaction even with steam
Mg	Very slow with cold water, reacts with steam
Ca, Sr, Ba	React readily, increasing vigor

$$\boxed{M + 2H_2O \rightarrow M(OH)_2 + H_2 \uparrow}$$

Why Be doesn’t react?

• Very small size → high ionization energy
• BeO protective layer forms

2. Reaction with Oxygen

$$\boxed{2M + O_2 \rightarrow 2MO}$$

All form normal oxides (MO, not peroxides)

Exception: BaO₂ can be formed under specific conditions

3. Reaction with Acids

$$\boxed{M + 2HCl \rightarrow MCl_2 + H_2 \uparrow}$$

Vigorous reaction with dilute acids

Anomalous Behavior of Beryllium

Important Compounds

Calcium Oxide (CaO) - Quick Lime

Preparation:

$$\boxed{CaCO_3 \xrightarrow{\Delta, 1200K} CaO + CO_2}$$

Properties:

• White solid
• Reacts vigorously with water (exothermic)

$$CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$$

Uses:

• Manufacturing cement
• Metallurgy (flux)
• Making slaked lime

Calcium Hydroxide - Ca(OH)₂ - Slaked Lime

Preparation:

$$CaO + H_2O \rightarrow Ca(OH)_2$$

Properties:

• Sparingly soluble (clear solution = lime water)
• Milky suspension = milk of lime
• Basic in nature

Lime water test for CO₂:

$$\boxed{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 \downarrow + H_2O}$$

(milky)

Excess CO₂:

$$CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$$

(soluble, milky clears)

Calcium Carbonate (CaCO₃)

Occurrence: Limestone, marble, chalk

Thermal decomposition:

$$\boxed{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}$$

Uses:

• Construction (marble, limestone)
• Cement manufacturing
• Antacid (Tums tablets)

Gypsum (CaSO₄·2H₂O)

Plaster of Paris:

$$\boxed{CaSO_4 \cdot 2H_2O \xrightarrow{393K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O}$$

Setting of PoP:

$$CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$$

(hardens)

Uses:

• Bone fracture casts
• Making molds
• Construction

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Comparison: Group 1 vs Group 2

Property	Group 1 (ns¹)	Group 2 (ns²)
Valence electrons	1	2
Common oxidation state	+1 only	+2 only
Reactivity	Very high	Moderate
Hardness	Soft (cut with knife)	Harder than Group 1
Melting point	Lower	Higher
Compounds	More ionic	Less ionic than Group 1
Hydration energy	Lower	Higher (M²⁺ ions)
Reaction with water	Violent	Moderate (except Be)

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Diagonal Relationship

Li ~ Mg

Property	Li	Mg	Why Similar?
Hardness	Harder than other Group 1	Hard	Similar size/charge ratio
Carbonate	Decomposes on heating	Decomposes	Both have high lattice energy
Nitrate	Gives NO₂	Gives NO₂	Similar polarizing power
Oxide	Forms Li₂O	Forms MgO	Both prefer normal oxide

$$Li_2CO_3 \xrightarrow{\Delta} Li_2O + CO_2$$ $$MgCO_3 \xrightarrow{\Delta} MgO + CO_2$$

(Other Group 1 carbonates don’t decompose easily!)

Be ~ Al

Property	Be	Al	Why Similar?
Chloride nature	Covalent	Covalent	High charge density
Oxide nature	Amphoteric	Amphoteric	Similar electronegativity
Complex formation	Forms complexes	Forms complexes	Small size, high charge

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Flame Test Colors

Group 1:

Element	Flame Color	Why?
Li	Crimson red	670 nm
Na	Golden yellow	589 nm (D-lines)
K	Lilac/violet	766 nm
Rb	Red-violet	780 nm
Cs	Blue	455 nm

Group 2:

Element	Flame Color
Ca	Brick red
Sr	Crimson red
Ba	Green

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Solubility Trends

Hydroxides

Group 1: All highly soluble

Group 2: Solubility increases down the group

• Be(OH)₂: Amphoteric, sparingly soluble
• Mg(OH)₂: Sparingly soluble (milk of magnesia)
• Ca(OH)₂: Slightly soluble (lime water)
• Sr(OH)₂, Ba(OH)₂: More soluble

Sulphates

Group 1: All highly soluble

Group 2: Solubility decreases down the group (opposite!)

• BeSO₄: Highly soluble
• MgSO₄: Soluble (Epsom salt)
• CaSO₄: Slightly soluble (gypsum)
• SrSO₄: Sparingly soluble
• BaSO₄: Insoluble (used in barium meal test)

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Common Mistakes to Avoid

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Practice Problems

Level 1: Foundation (NCERT)

Level 2: JEE Main

Level 3: JEE Advanced

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Quick Revision Box

Topic	Key Points
Group 1 oxides	Li→Li₂O, Na→Na₂O₂, K/Rb/Cs→MO₂
Reactivity with H₂O	Increases down group (both Group 1 & 2)
Be anomaly	Doesn’t react with H₂O, covalent compounds, amphoteric
Diagonal pairs	LiMg, BeAl
Lime water test	CO₂ → milky (CaCO₃), excess → clear (Ca(HCO₃)₂)
Gypsum	CaSO₄·2H₂O
Plaster of Paris	CaSO₄·½H₂O
Solubility (Group 2)	Hydroxides ↑, Sulphates ↓ (down group)
Flame test	Na=Yellow, K=Lilac, Ca=Brick red, Ba=Green

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When to Use This

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JEE Strategy: High-Yield Points

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Teacher’s Summary

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Related Topics

Prerequisites

• Modern Periodic Law - Understand periodic table structure
• Periodic Trends - IE, reactivity patterns
• Electronic Configuration - Why ns¹ and ns²

Next Steps

• p-block Introduction - Compare with Groups 13-18
• Chemical Bonding - Why s-block forms ionic compounds
• d-block Elements - Contrast with transition metals

Applications

• Redox Reactions - s-block metals as reducing agents
• Electrochemistry - Reactivity series
• Solutions - Solubility products of carbonates, hydroxides

Cross-Subject Connections

• Electrolysis - Extraction of Na, Mg from molten salts
• Thermodynamics - Lattice energy vs hydration energy

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