Alkaline Earth Metals: Properties, Trends & Uses

TAKEAWAYS:

• Alkaline earth metals are Group 2 elements with 2 valence electrons (ns²) and form +2 ions in compounds.
• Their reactivity increases down the group (except Be), with atomic size, metallic character, and basicity of oxides growing stronger.
• Calcium is the most biologically important (bones, teeth, milk), while magnesium is vital for chlorophyll and enzymes.
• Extraction varies: electrolysis for Mg, thermal decomposition for CaCO₃, and roasting for ZnS (impurity).
• Uses range from Be in aerospace alloys to Ba in X-ray contrast agents and Sr in fireworks.
• Exceptions: Beryllium behaves differently due to its small size and high ionization energy.

What Are Alkaline Earth Metals?

Alkaline earth metals are the six elements in Group 2 of the periodic table: Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra). They are called "alkaline" because their oxides and hydroxides are basic (alkaline) when dissolved in water. They are called "earth" because their oxides were once thought to be "earths" (impure oxides).

Key Properties:

Electronic Configuration and Oxidation State

All Group 2 elements have 2 electrons in their outermost s-orbital (ns²). When they react, they lose these 2 electrons to form +2 cations (M²⁺). Example:

• Magnesium (Mg): 1s² 2s² 2p⁶ 3s² → Loses 2e⁻ → Mg²⁺ (stable noble gas config: 1s² 2s² 2p⁶).
• Calcium (Ca): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² → Loses 2e⁻ → Ca²⁺.

Why +2 only?

• The noble gas configuration after losing 2e⁻ is very stable.
• The ionization energy for losing a 3rd electron is extremely high (e.g., Mg²⁺ → Mg³⁺ requires ~7700 kJ/mol vs. 738 kJ/mol for Mg → Mg²⁺).

Physical Properties

1. Appearance and State

• All are silvery-white metals (except Be, which is grayish).
• Soft (can be cut with a knife, like Na), but harder than Group 1 metals.
• Mg, Ca, Sr, Ba are softer than Al but harder than Na/K.

2. Density

• Be and Mg are lighter than water (used in aerospace).
• Ca, Sr, Ba are denser than water (Ba is ~3.5x denser than water).

3. Melting and Boiling Points

• Higher than Group 1 (stronger metallic bonding).
• Mg has the highest MP (650°C) in the group, while Ba has the lowest (727°C).

Chemical Properties

1. Reaction with Air (Oxidation)

• Burn in air to form oxides (M₂O) or hydroxides (M(OH)₂).
• Be forms a protective oxide layer (like Al), so it does not react further.
• Mg, Ca, Sr, Ba burn brightly in air:
  • Mg: Dazzling white flame (used in flares).
  • Ca: Brick-red flame (used in fireworks).
  • Sr: Crimson flame (fireworks).
  • Ba: Apple-green flame (fireworks).

2. Reaction with Water

• Be → No reaction (too unreactive).
• Mg → Very slow reaction (forms a protective hydroxide layer).
• Ca, Sr, Ba → React vigorously to form hydroxide + hydrogen gas: $$\text{Ca}+2{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2+{\text{H}}_2$$

3. Reaction with Dilute Acids

• All react with dilute HCl/H₂SO₄ to form salts + hydrogen gas: $$\text{Mg}+2\text{HCl}\to {\text{MgCl}}_2+{\text{H}}_2$$
• Order of reactivity: Ba > Sr > Ca > Mg > Be (Be is almost inert).

4. Reaction with Halogens

• Form halides (MX₂): $$\text{Mg}+{\text{Cl}}_2\to {\text{MgCl}}_2$$
• BeCl₂ is covalent (due to small size and polarization), while others are ionic.

Trends Down the Group

1. Atomic and Ionic Size

• Increases down the group (more electron shells).
• Ionic radius (M²⁺) increases (e.g., Be²⁺ < Mg²⁺ < Ca²⁺).

2. Ionization Energy

• Decreases down the group (outer e⁻ are farther from the nucleus).
• Be has the highest IE (small size, strong nuclear attraction).

3. Electronegativity

• Decreases down the group (Be is most electronegative in the group).

4. Reactivity

• Increases down the group (easier to lose e⁻).
• Be is least reactive, Ba is most reactive.

5. Basicity of Oxides and Hydroxides

• BeO and Be(OH)₂ are amphoteric (react with both acids and bases).
• MgO and Mg(OH)₂ are weakly basic.
• CaO, SrO, BaO are strongly basic (react vigorously with water to form hydroxides).

flowchart TD
    A["BeO/Be(OH)₂"] -->|"Amphoteric"| B["React with acids & bases"]
    C["MgO/Mg(OH)₂"] -->|"Weakly basic"| D["React with acids only"]
    E["CaO/SrO/BaO"] -->|"Strongly basic"| F["React vigorously with water"]

Extraction of Alkaline Earth Metals

1. Beryllium (Be)

• Found in beryl (3BeO·Al₂O₃·6SiO₂) and bertrandite (Be₄Si₂O₇(OH)₂).
• Extracted by:
  1. Roasting beryl with sodium fluoride to form BeF₂.
  2. Electrolysis of molten BeF₂ (very high MP, so mixed with NaF/KF to lower MP).

2. Magnesium (Mg)

• Main sources: Seawater (Mg²⁺ ions), dolomite (CaCO₃·MgCO₃), carnallite (KCl·MgCl₂·6H₂O).
• Extraction methods:
  • Electrolysis of molten MgCl₂ (from seawater or carnallite).
  • Thermal decomposition of MgO (from seawater): $$\text{MgO}+\text{C}\to \text{Mg}+\text{CO}$$

3. Calcium (Ca)

• Main source: Limestone (CaCO₃).
• Extracted by:
  1. Thermal decomposition of CaCO₃: $${\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2$$
  2. Electrolysis of molten CaCl₂ (from limestone + HCl).

4. Strontium (Sr) and Barium (Ba)

• Extracted from celestite (SrSO₄) and barite (BaSO₄).
• Process:
  1. Roast with carbon to form sulfides: $${\text{SrSO}}_4+4\text{C}\to \text{SrS}+4\text{CO}$$
  2. React with water to form hydroxides: $$\text{SrS}+2{\text{H}}_2\text{O}\to {\text{Sr(OH)}}_2+{\text{H}}_2\text{S}$$
  3. Electrolyze molten hydroxide to get Sr/Ba.

Uses of Alkaline Earth Metals

Biological Importance

1. Calcium (Ca)

• Bones and teeth: Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) gives strength.
• Blood clotting: Ca²⁺ ions are essential.
• Muscle contraction: Ca²⁺ triggers actin-myosin interactions.
• Milk: Contains Ca(H₂PO₄)₂ and CaCO₃.

2. Magnesium (Mg)

• Chlorophyll: Central Mg²⁺ in chlorophyll helps photosynthesis.
• Enzymes: Activates ATPase (energy transfer).
• Nerves: Regulates Na⁺/K⁺ pumps.
• Seawater: ~0.5% Mg²⁺ (3rd most abundant ion in seawater).

Anomalous Behavior of Beryllium

Beryllium differs from other Group 2 elements due to:

1. Small size (similar to Al).
2. High ionization energy (hard to lose e⁻).
3. Covalent character in compounds (e.g., BeCl₂ is linear and covalent).

Solved Examples

Example 1: Writing Equations

Q: Write the balanced equation for the reaction of calcium with cold water. A: $$\text{Ca}+2{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2+{\text{H}}_2$$

• Observation: Effervescence (H₂ gas) and a milky solution (Ca(OH)₂).

Example 2: Trend Analysis

Q: Explain why the reactivity of alkaline earth metals increases down the group. A:

1. Atomic size increases → outer e⁻ are farther from the nucleus.
2. Nuclear attraction decreases → e⁻ are easier to lose.
3. Ionization energy decreases → less energy needed to remove e⁻.
4. Hydration energy of M²⁺ ions becomes more exothermic (larger ions stabilize better in water).

Example 3: Extraction Calculation

Q: How much CaO is needed to produce 500 kg of Ca from CaCO₃? A:

1. Decomposition reaction: $${\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2$$
2. Molar masses:
   • Ca = 40 g/mol, CaO = 56 g/mol, CaCO₃ = 100 g/mol.
3. Moles of Ca needed: $$\text{Moles of Ca}=\frac{500,000\text{ g}}{40\text{ g/mol}}=12,500\text{ mol}$$
4. Moles of CaO produced (1:1 ratio): $$12,500\text{ mol CaO}$$
5. Mass of CaO: $$12,500\text{ mol}\times 56\text{ g/mol}=700,000\text{ g}=700\text{ kg}$$

NEB Board-Style Questions

Short Answer Questions (2 marks each)

1. Why is beryllium less reactive than magnesium?

   • Answer: Be has a smaller atomic size and higher ionization energy, making it harder to lose electrons.

2. Write the equation for the reaction of magnesium with steam.

   • Answer: $$\text{Mg}+{\text{H}}_2\text{O (steam)}\to \text{MgO}+{\text{H}}_2$$

3. Name two uses of calcium oxide.

   • Answer: (i) Manufacture of cement, (ii) Bleaching powder production.

4. Why does barium sulfate act as an X-ray contrast agent?

   • Answer: BaSO₄ is insoluble and opaque to X-rays, so it shows up clearly in medical imaging.

5. Complete the reaction: Sr + H₂O → ?

   • Answer: $$\text{Sr}+2{\text{H}}_2\text{O}\to {\text{Sr(OH)}}_2+{\text{H}}_2$$

Long Answer Questions (5 marks each)

6. Describe the extraction of magnesium from seawater. Give balanced equations.

   • Answer:
     1. Precipitation: $$\text{Mg²⁺ (aq)}+{\text{Ca(OH)}}_2\to {\text{Mg(OH)}}_2\downarrow +\text{Ca²⁺ (aq)}$$
     2. Dehydration: $${\text{Mg(OH)}}_2\to \text{MgO}+{\text{H}}_2\text{O}$$
     3. Reduction: $$\text{MgO}+\text{C}\to \text{Mg}+\text{CO}$$
     4. Electrolysis (alternative): $${\text{MgCl}}_2\to \text{Mg}+{\text{Cl}}_2$$

7. Explain the trend in reactivity of alkaline earth metals down the group. Why is beryllium an exception?

   • Answer:
     • Trend: Reactivity increases due to increasing atomic size, decreasing IE, and better hydration of larger M²⁺ ions.
     • Be exception: Small size, high IE, and covalent bonding make it less reactive and amphoteric.

8. How is calcium used in the following?

   • (i) Cement manufacture
     • Answer: CaO (quicklime) reacts with SiO₂ to form calcium silicates (cement).
   • (ii) Bleaching powder
     • Answer: CaO reacts with Cl₂ and H₂O to form CaOCl₂ (bleaching powder).
   • (iii) Food industry
     • Answer: Calcium phosphate (E450) is used as a food additive.

Practical/Numerical Questions (3-4 marks)

9. Calculate the mass of magnesium oxide formed when 12 g of magnesium burns in excess air.

   • Solution:
     1. Moles of Mg: $$\frac{12\text{ g}}{24\text{ g/mol}}=0.5\text{ mol}$$
     2. Equation: $$2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$$
     3. Moles of MgO = 0.5 mol (1:1 ratio).
     4. Mass of MgO: $$0.5\text{ mol}\times 40\text{ g/mol}=20\text{ g}$$

10. A sample of strontium carbonate decomposes to give 20 g of SrO. Calculate the mass of CO₂ released.

    • Solution:
      1. Moles of SrO: $$\frac{20\text{ g}}{104\text{ g/mol}}\approx 0.192\text{ mol}$$
      2. Equation: $${\text{SrCO}}_3\to \text{SrO}+{\text{CO}}_2$$
      3. Moles of CO₂ = 0.192 mol.
      4. Mass of CO₂: $$0.192\text{ mol}\times 44\text{ g/mol}\approx 8.45\text{ g}$$

Exam Tip

What to Focus On:

1. Trends: Always explain trends in reactivity, atomic size, IE, and basicity down the group.
2. Equations: Know reactions with water, acids, halogens, and air for each metal.
3. Extraction: Memorize how Mg, Ca, and Be are extracted (electrolysis vs. thermal methods).
4. Uses: Link metal properties to uses (e.g., Mg’s lightness → aerospace, Ca’s basicity → cement).
5. Anomalies: Beryllium is different—highlight its small size, covalent compounds, and amphoteric oxides.
6. Calculations: Practice stoichiometry (e.g., mass of product from reactant mass).

Common Mistakes to Avoid:

• Forgetting Be’s exceptions (e.g., writing BeO as basic instead of amphoteric).
• Incorrect equations (e.g., writing Ca + H₂O → CaO + H₂ instead of Ca(OH)₂ + H₂).
• Mixing Group 1 and Group 2 trends (Group 2 is less reactive than Group 1).
• Ignoring units in numerical problems (always convert g → mol → g).

Quick Revision Table:

Final Advice: Draw trend diagrams, reaction flowcharts, and label periodic table positions—NEB loves visual answers! Practice 1-2 numerical problems per week to master stoichiometry. Good luck! 🚀