Introduction to Group 2 Elements
Group 2 elements, also known as alkaline earth metals, include Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra). These elements are called “alkaline earth metals” because their oxides and hydroxides are alkaline (basic) in nature and are found in the Earth’s crust.
Be
Beryllium
Atomic No: 4
Mg
Magnesium
Atomic No: 12
Ca
Calcium
Atomic No: 20
Sr
Strontium
Atomic No: 38
Ba
Barium
Atomic No: 56
Ra
Radium
Atomic No: 88
Key Characteristics
General electronic configuration: ns²
All have two valence electrons in their outermost shell
Form +2 ions by losing both valence electrons
Good reducing agents as they get oxidized
Reactivity increases down the group
Form ionic compounds with non-metals
Important Note
Beryllium (Be) shows different behavior compared to other Group 2 elements due to its small size and high charge density. It forms covalent compounds and shows amphoteric behavior, unlike other members which form predominantly ionic compounds.
Electronic configurations:
Be: 1s² 2s² Mg: 1s² 2s² 2p⁶ 3s² Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Be: 1s² 2s² Mg: 1s² 2s² 2p⁶ 3s² Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Physical Properties and Trends
Trends in Physical Properties
Atomic Radius: Increases down the group due to additional electron shells
Ionization Energy: Decreases down the group due to increased shielding and atomic size
Melting Point: Generally decreases down the group (except for irregularity at Mg)
Density: Increases down the group
| Element | Atomic Radius (nm) | 1st Ionization Energy (kJ/mol) | Melting Point (°C) | Density (g/cm³) |
|---|---|---|---|---|
| Beryllium (Be) | 0.122 | 900 | 1280 | 1.85 |
| Magnesium (Mg) | 0.160 | 738 | 650 | 1.74 |
| Calcium (Ca) | 0.197 | 590 | 838 | 1.55 |
| Strontium (Sr) | 0.215 | 549 | 768 | 2.66 |
| Barium (Ba) | 0.217 | 503 | 714 | 3.56 |
Tips & Tricks
Remember the atomic radius trend: Be < Mg < Ca < Sr < Ba
Melting point decreases down the group due to weaker metallic bonding as atomic size increases
Ionization energy decreases because outer electrons are farther from nucleus and experience more shielding
Beryllium has unusually high melting point due to its small size and strong metallic bonding
Atomic Radius Trend
Be → Mg → Ca → Sr → Ba
Increasing atomic radius
Chemical Reactivity
Reactivity Trends
Reactivity increases down the group
Due to decreasing ionization energy
Barium is so reactive it must be stored under oil
Beryllium shows anomalous behavior due to small size
Reactions with Oxygen
2Mg(s) + O₂(g) → 2MgO(s) (Bright white flame)
All Group 2 metals burn in air to form white solid oxides
Strontium and Barium also form peroxides (MO₂)
BeO is amphoteric, others are basic
Basic character increases down the group
Reactions with Water
Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
Beryllium: No reaction (protective oxide layer)
Magnesium: Reacts with steam only
Calcium onwards: React with cold water with increasing vigor
Produces metal hydroxide and hydrogen gas
Reactions with Acids
M(s) + 2HCl(aq) → MCl₂(aq) + H₂(g)
All Group 2 metals react with dilute acids
Reactions become more vigorous down the group
Redox reactions producing salts and hydrogen gas
Memorization Strategy
Create a reactivity series: Be (least) < Mg < Ca < Sr < Ba (most reactive)
Remember flame test colors: Ca (brick-red), Sr (scarlet/red), Ba (apple-green)
Practice writing balanced equations for each reaction type
Note exceptions: Be doesn’t react with water, Mg reacts only with steam
Complexes of Alkaline Earth Metals
Group 2 elements almost exclusively form ionic compounds containing the M²⁺ ion. They are more reactive towards group 5 elements, and they have a greater tendency to form complexes with Lewis bases than do the alkali metals.
Complex Formation Trends
Higher tendency to form complexes than alkali metals due to +2 charge and smaller ionic radii
Complex formation tendency decreases down the group
Beryllium forms the most complexes due to its small size and high charge density
Be²⁺ behaves as a strong Lewis acid
Beryllium Complexes
[Be(H₂O)₄]²⁺ (Tetrahedral aqua complex)
[Be(OH)₄]²⁻ (Tetrahedral hydroxo complex)
[BeF₄]²⁻ (Very stable tetrahedral fluoride complex)
Important Note
Because of its high charge-to-radius ratio, the Be²⁺ ion polarizes coordinated water molecules, increasing their acidity. This explains why beryllium salts form acidic solutions in water.
Complexes of Heavier Alkaline Earth Metals
Mg²⁺ and Ca²⁺ form complexes with coordination number 6 or higher
Aqueous solutions contain octahedral [Mg(H₂O)₆]²⁺ ions
Mg can extend coordination number to six using 3s, three 3p and two 3d orbitals
Complex formation is most important for smaller cations (Mg²⁺ and Ca²⁺)
Memory Aid
“Be forms tetrahedral complexes, others prefer octahedral”
Complex stability: Be > Mg > Ca > Sr > Ba
Remember: Be²⁺ polarizes water molecules making solutions acidic
Be(OH)₂ is amphoteric – forms [Be(OH)₄]²⁻ in strong base
Extraction and Purification Processes
General Extraction Process
Ore Preparation: Mining and crushing minerals (carbonates, sulfates, oxides, silicates)
Roasting: Thermal decomposition of carbonates to oxides
Reduction: Electrolysis or chemical reduction to obtain pure metal
Purification: Further refining through various methods
Ore Preparation
Mining and crushing
Roasting
Thermal decomposition
Reduction
Electrolysis or chemical reduction
Purification
Refining processes
Roasting Process
MgCO₃(s) → MgO(s) + CO₂(g) (Heating)
Group 2 carbonates are unstable towards heat and thermally decompose to form metal oxides and carbon dioxide.
Specific Extraction Methods
Beryllium Extraction
Extracted from beryl ore (Be₃Al₂(SiO₃)₆)
Chemical process involving acid digestion and solvent extraction
Purification: Convert Be(OH)₂ to BeF₂, then reduce with magnesium
Magnesium Extraction
Produced by electrolysis of molten magnesium chloride (from sea water or brines)
Purification: Fractional distillation or vacuum distillation
Calcium, Strontium, and Barium
Obtained by reducing their halides with sodium or magnesium
Pure samples obtained by electrolysis of chlorides or oxides
Extraction Tips
Remember: Electrolysis is common for purification
Beryllium requires special chemical processes due to its unique properties
Magnesium from sea water – important industrial source
Thermal decomposition of carbonates is the first step for many extraction processes
Important Safety Note
Radium chloride, being radioactive, requires special handling and separation procedures during extraction and purification processes.
Comparison with Group 1 Elements
| Property | Group 1 (Alkali Metals) | Group 2 (Alkaline Earth Metals) |
|---|---|---|
| Valence Electrons | One (ns¹) | Two (ns²) |
| Common Ion | M⁺ (Monovalent) | M²⁺ (Divalent) |
| Ionization Energy | Lower | Higher |
| Reactivity with Water | Very vigorous | Less vigorous |
| Melting Point | Lower | Higher |
| Hydroxide Solubility | All highly soluble | Increases down the group |
| Carbonate Stability | Do not decompose on heating | Decompose to oxide and CO₂ |
| Complex Formation | Limited complex formation | Significant complex formation, especially for smaller ions |
Key Difference
Group 2 elements are less reactive than Group 1 elements in the same period because Group 2 elements have higher ionization energies (need to remove two electrons instead of one). Additionally, Group 2 elements form more stable complexes due to their higher charge density.
Comparison Memory Aid
Group 2 has higher melting points and ionization energies
Group 2 has lower reactivity with water
Group 2 forms +2 ions vs Group 1’s +1 ions
Group 2 forms more complexes due to higher charge density
Thermal Stability of Carbonates and Nitrates
Thermal Decomposition
Thermal stability increases down the group
Due to decreasing polarizing power of cations
Smaller cations polarize anions more effectively
Easier polarization = easier decomposition
Carbonate Decomposition
MgCO₃(s) → MgO(s) + CO₂(g) (at 540°C)
Nitrate Decomposition
2Ca(NO₃)₂(s) → 2CaO(s) + 4NO₂(g) + O₂(g)
| Compound | Decomposition Temperature (°C) | Enthalpy Change (kJ/mol) |
|---|---|---|
| MgCO₃ | 540 | +117 |
| CaCO₃ | 900 | +176 |
| SrCO₃ | 1280 | +238 |
| BaCO₃ | 1360 | +268 |
Thermal Stability Trend
MgCO₃ → CaCO₃ → SrCO₃ → BaCO₃
Increasing thermal stability
Understanding Polarization
Small M²⁺ ions (like Mg²⁺) have high charge density
They strongly polarize large anions (CO₃²⁻, NO₃⁻)
This distortion weakens the anion structure
Weaker structure = easier thermal decomposition
Solubility Trends
Solubility Patterns
Hydroxides: Solubility increases down the group
Sulfates: Solubility decreases down the group
Explained by lattice energy and hydration energy balance
Hydroxide Solubility
Order: Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂
Sulfate Solubility
Order: MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄
| Compound | Solubility (mol/100g water at 298K) |
|---|---|
| Mg(OH)₂ | 2 × 10⁻⁵ |
| Ca(OH)₂ | 1.5 × 10⁻³ |
| Sr(OH)₂ | 3.4 × 10⁻³ |
| Ba(OH)₂ | 1.5 × 10⁻² |
Solubility Trends
Hydroxides: Mg(OH)₂ → Ba(OH)₂
Increasing solubility
Sulfates: MgSO₄ → BaSO₄
Decreasing solubility
Memory Aid
“Hydroxides get more soluble, Sulfates get less soluble”
Ba(OH)₂ is soluble, BaSO₄ is insoluble (used in medicine)
Remember exceptions: Be compounds often show different behavior
Applications and Uses
Industrial and Medical Uses
Limestone (CaCO₃): Building material, cement production
Slaked Lime (Ca(OH)₂): Soil treatment for acidic soils
Barium Sulfate (BaSO₄): X-ray contrast medium
Magnesium Hydroxide (Mg(OH)₂): Antacid (Milk of Magnesia)
Calcium Carbonate (CaCO₃): Antacid, calcium supplement
Beryllium: Aerospace applications, X-ray windows
Magnesium: Alloys, pyrotechnics (bright white light)
Medical Applications
Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l) (Antacid reaction)
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l) (Antacid reaction)
Important Medical Note
Barium sulfate is used in medicine for X-ray imaging because it’s insoluble and not absorbed into the bloodstream, making it safe for internal use despite barium being toxic in soluble forms.
Application Memory Tips
Remember: Mg and Ca compounds are used in medicine as antacids
Calcium compounds are essential for construction materials
Magnesium produces bright white light in fireworks
Barium sulfate is safe for X-rays because it’s insoluble