Group 2 Elements (Alkaline Earth Metals)
| Element | Symbol | Atomic No. | Atomic Mass (g mol⁻¹) | Electron Configuration | Common Oxidation State |
|---|---|---|---|---|---|
| Beryllium | Be | 4 | 9.01 | [He] 2s² | +2 |
| Magnesium | Mg | 12 | 24.31 | [Ne] 3s² | +2 |
| Calcium | Ca | 20 | 40.08 | [Ar] 4s² | +2 |
| Strontium | Sr | 38 | 87.62 | [Kr] 5s² | +2 |
| Barium | Ba | 56 | 137.33 | [Xe] 6s² | +2 |
| Radium | Ra | 88 | 226.00 (unstable) | [Rn] 7s² | +2 |
Tip: All Group 2 elements have a 2s² + 2s² configuration (two valence electrons) and form divalent cations (M²⁺).
1. Simple Explanation
Group 2 (alkaline earth metals) are the second column of the periodic table. They share the following traits:
- Valence electrons: 2 (in the outermost s‑orbital).
- Oxidation state: Almost always +2.
- Reactivity: Increasing with atomic number (Be < Mg < Ca < Sr < Ba < Ra).
- Physical traits: Dense, silvery, relatively soft metals.
- Chemical traits: Form strongly basic hydroxides, basic oxides, and soluble chlorides (except BeCl₂).
These metals are “earthy” in the sense that they are found in the Earth’s crust (hence earth metals).
2. Key Points
| Category | Details |
|---|---|
| Atomic Size | Increases down the group (Be < Mg < Ca < Sr < Ba). |
| Electronegativity | Decreases down the group. |
| First & Second Ionization Energies | Low compared to Group 1; second IE rises sharply (needs to remove the second electron from a stable noble‑gas core). |
| Melting / Boiling Points | Decline down the group. |
| Solubility of Oxides/Hydroxides | Basic oxides are sparingly soluble; hydroxides are only slightly soluble (except Be(OH)₂). |
| Common Compounds | • Oxides (BeO, MgO, CaO, SrO, BaO) – basic, lattice energies high. • Hydroxides (Be(OH)₂, Mg(OH)₂, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂) – soluble in acid. • Chlorides & Bromides – soluble, except BeCl₂ (forms covalent polymer). • Carbonates – sparingly soluble (except CaCO₃, BaCO₃). |
| Reactivity with Water | • Mg, Ca, Sr, Ba react slowly with cold water. • Warm water → vigorous reaction (Ca, Sr, Ba). • Be and Mg react only with steam. |
| Uses | • Be – aerospace alloys, x‑ray windows. • Mg – fireworks, magnesium alloy (lightweight), in photosensitizers. • Ca – calcium carbonate (chalk), CaO (quicklime), Ca(OH)₂ (slaked lime). • Sr – fireworks (red color), SrTiO₃ in electronics. • Ba – barometric pressure gauges, glass manufacturing, MRI contrast agents. |
| Toxicity / Health | Be is a potent lung carcinogen; Ba is mildly toxic. Others are generally safe in small amounts. |
| Stability | React strongly with acids, halogens, and oxygen. They are oxidized to their +2 state in most reactions. |
3. Worked Example
Problem
Predict the products and write a balanced equation for the reaction of calcium metal with hydrogen chloride gas. Then calculate the mass of Ca needed to produce 15.0 g of CaCl₂.
Step 1 – Identify the reaction type
- Calcium (a reactive metal) + hydrogen chloride (an acid) → salt + hydrogen gas.
- Reaction type: Displacement / single‑replacement.
Step 2 – Balance the equation
Ca (s) + 2 HCl (g) → CaCl₂ (s) + H₂ (g)
Step 3 – Molar masses
- Ca = 40.08 g mol⁻¹
- CaCl₂ = 110.98