Group I and Group II Elements: S-Block Metals
Group I (alkali metals) and Group II (alkaline earth metals) elements are collectively known as the s-block elements because their valence electrons are in the s orbital. These groups show interesting trends and important chemistry.
Group I: The Alkali Metals
Definition: Group I elements are called alkali metals because they form alkaline (basic) solutions when they react with water.
Electronic Configuration: ns¹ (one electron in the outer s orbital)
Elements: Li, Na, K, Rb, Cs, Fr
Note: Hydrogen is also in Group I but is NOT an alkali metal.
Physical Properties of Group I
- Soft metals (can be cut with a knife)
- Low density (Li, Na, K float on water)
- Low melting and boiling points
- Good conductors of heat and electricity
- Metallic luster (shiny appearance)
- Highly reactive
Trends Down Group I
| Property | Li | Na | K | Rb | Cs | Trend |
|---|---|---|---|---|---|---|
| Atomic Radius (nm) | 0.15 | 0.19 | 0.23 | 0.25 | 0.26 | Increases |
| First IE (kJ/mol) | 520 | 500 | 420 | 400 | 380 | Decreases |
| Melting Point (°C) | 180 | 98 | 63 | 39 | 29 | Decreases |
| Reactivity | Low | Moderate | High | Very High | Extremely High | Increases |
Chemical Properties of Group I
1. Reaction with Water
2M + 2H₂O → 2MOH + H₂↑
Reactivity increases down the group: Li reacts gently, Cs reacts explosively
2. Reaction with Oxygen
- Li forms: Li₂O (lithium oxide) – normal oxide
- Na forms: Na₂O₂ (sodium peroxide) – peroxide
- K and heavier: Form superoxides (KO₂)
3. Reaction with Halogens
2M + Cl₂ → 2MCl
4. All Group I metals form +1 ions
M → M⁺ + e⁻
Why? They have one valence electron which is easily lost to achieve a stable noble gas configuration.
Group II: The Alkaline Earth Metals
Definition: Group II elements are called alkaline earth metals because they form alkaline solutions and were originally called “earth” oxides.
Electronic Configuration: ns² (two electrons in the outer s orbital)
Elements: Be, Mg, Ca, Sr, Ba, Ra
Physical Properties of Group II
- Harder than Group I metals but still relatively soft
- Higher density than Group I metals
- Higher melting and boiling points than Group I
- Good conductors of heat and electricity
- Metallic luster
- Less reactive than Group I metals
Trends Down Group II
| Property | Be | Mg | Ca | Sr | Ba | Trend |
|---|---|---|---|---|---|---|
| Atomic Radius (nm) | 0.11 | 0.16 | 0.20 | 0.21 | 0.22 | Increases |
| First IE (kJ/mol) | 900 | 740 | 590 | 550 | 500 | Decreases |
| Melting Point (°C) | 1280 | 650 | 660 | 770 | 710 | Generally High |
| Reactivity | Low | Moderate | High | Very High | Extremely High | Increases |
Chemical Properties of Group II
1. Reaction with Water
M + 2H₂O → M(OH)₂ + H₂↑
Note: Be and Mg react very slowly; Ca reacts readily; Sr and Ba react vigorously
2. Reaction with Oxygen
2M + O₂ → 2MO
All Group II elements form normal oxides (MO), unlike Group I
3. Reaction with Halogens
M + Cl₂ → MCl₂
4. All Group II metals form +2 ions
M → M²⁺ + 2e⁻
Why? They have two valence electrons which are easily lost to achieve a stable noble gas configuration.
Differences Between Group I and Group II
| Property | Group I (Alkali Metals) | Group II (Alkaline Earth Metals) |
|---|---|---|
| Valence Electrons | One (s¹) | Two (s²) |
| Ion Formed | M⁺ | M²⁺ |
| Reactivity | More reactive | Less reactive |
| Melting Point | Low | Relatively higher |
| Hardness | Very soft | Harder |
| Oxides | Normal oxides (M₂O), Peroxides (M₂O₂), Superoxides (MO₂) | Normal oxides (MO) |
| Solubility of Hydroxides | All soluble | Increasing solubility down group |
| Solubility of Carbonates | Soluble | Insoluble except Ba(CO₃) slightly soluble |
Special Properties of Some S-Block Elements
Lithium (Li)
Despite being in Group I, lithium has some unusual properties:
- Smallest Group I metal
- Highest melting point of Group I
- Forms only normal oxide (Li₂O), not peroxide
- Shows diagonal relationship with Mg (similar size and polarizing power)
Beryllium (Be)
Beryllium is unique among Group II elements:
- Smallest Group II metal
- High ionisation energy (closer to Al than to Mg)
- Forms covalent compounds more readily
- Compounds often show covalent character
- Shows diagonal relationship with Al
Magnesium (Mg)
- Most important Group II metal industrially
- Used in alloys, construction, medicine
- Essential for biological processes (chlorophyll)
- Forms ionic compounds with typical ionic properties
Solubility Trends in S-Block Salts
Hydroxides: Solubility increases down Group II (Be(OH)₂ insoluble → Ba(OH)₂ soluble)
Sulfates: Solubility decreases down Group II (MgSO₄ soluble → BaSO₄ insoluble)
Carbonates: Solubility decreases down Group II (MgCO₃ slightly soluble → BaCO₃ insoluble)
Uses of S-Block Elements and Compounds
Lithium: Batteries, alloys, psychiatric medication
Sodium: NaCl, NaOH, Na₂CO₃ (industrial chemicals)
Potassium: KCl, K₂SO₄ (fertilizers)
Calcium: CaCO₃ (limestone), Ca(OH)₂ (slaked lime), CaSO₄ (plaster)
Magnesium: Alloys, Mg(OH)₂ (antacid), structural materials
Barium: BaSO₄ (medical imaging), Ba(NO₃)₂ (fireworks)
Summary: Key Characteristics of S-Block Elements
- Most reactive metals
- Form positive ions easily
- Reactivity increases down each group
- Group I forms +1 ions; Group II forms +2 ions
- Strong reducing agents
- Must be stored under mineral oil or argon
- Wide industrial and biological importance
