Barium (Ba) - Comprehensive Study Guide for CBSE, JEE, NEET
Introduction
Barium (Ba) is an alkaline earth metal with significant applications ranging from medical diagnostics to pyrotechnics. Its chemical reactivity and properties are characteristic of Group 2 elements, making it an important topic for understanding periodic trends and inorganic chemistry.
CBSE/JEE Quick Revision Notes
- Atomic Number: 56
- Atomic Mass: 137.33 u
- Group: 2 (Alkaline Earth Metals)
- Period: 6
- Block: s-block
- Valency: +2 (forms Ba²⁺ ion)
- Nature: Soft, silvery-white, reactive metal.
- Electronegativity (Pauling): 0.89
- Melting Point: 727 °C
- Boiling Point: 1897 °C
- Density: 3.59 g/cm³
- Key Characteristic: Shows characteristic apple-green flame in flame test.
Electron Configuration & Bonding Behavior
Electron Configuration
- Full Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s²
- Condensed Configuration: [Xe] 6s²
Bonding Behavior
- Barium readily loses its two outermost
6selectrons to achieve a stable noble gas configuration (that of Xenon, Xe). - It forms primarily ionic bonds, typically existing as the Ba²⁺ ion in compounds.
- Its electropositive nature makes it a strong reducing agent.
Crucial Chemical Reactions
1. Reaction with Air/Oxygen
Barium tarnishes rapidly in air and burns in oxygen to form barium oxide. With excess oxygen, it can form barium peroxide.
- Formation of Barium Oxide:
2Ba(s) + O₂(g) → 2BaO(s) - Formation of Barium Peroxide (with excess O₂):
Ba(s) + O₂(g) → BaO₂(s)
2. Reaction with Water
Barium reacts vigorously with water, evolving hydrogen gas and forming barium hydroxide. The reaction is less violent than that of alkali metals but more vigorous than lighter alkaline earth metals.
Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)
3. Reaction with Acids
Barium reacts readily with dilute acids (e.g., HCl) to produce hydrogen gas and the corresponding barium salt. With sulfuric acid, the reaction is limited by the formation of insoluble barium sulfate.
Ba(s) + 2HCl(aq) → BaCl₂(aq) + H₂(g)
4. Reaction with Halogens
Barium reacts directly with halogens to form barium halides.
Ba(s) + Cl₂(g) → BaCl₂(s)
5. Flame Test
Barium compounds impart a characteristic apple-green color to a non-luminous flame due to the excitation and subsequent de-excitation of its electrons. This is a common test for barium ions.
6. Formation of Insoluble Salts
Barium forms several important insoluble salts, which are crucial for qualitative analysis and industrial applications.
- Barium Sulfate (BaSO₄): Insoluble in water and dilute acids. Used as a confirmatory test for sulfate ions and in medical imaging.
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) - Barium Carbonate (BaCO₃): Insoluble in water.
Ba²⁺(aq) + CO₃²⁻(aq) → BaCO₃(s) - Barium Chromate (BaCrO₄): Insoluble, yellow precipitate.
Ba²⁺(aq) + CrO₄²⁻(aq) → BaCrO₄(s)
Industrial and Biological Importance
Industrial Importance
- Drilling Fluids: Barium sulfate (barite) is used as a weighting agent in oil and gas drilling muds.
- Glass and Ceramics: Barium compounds are incorporated into glass to increase its refractive index and into ceramics to improve their gloss.
- Pyrotechnics: Barium nitrate and chlorate are used to produce the characteristic apple-green color in fireworks.
- Vacuum Tubes: Barium is used as a getter in vacuum tubes to remove residual gases.
- Paints: Barium sulfate (blanc fixe) is used as a white pigment and filler.
Biological Importance
- Barium Meal (BaSO₄): Barium sulfate, being insoluble and non-toxic, is orally administered as a “barium meal” or enema for X-ray imaging of the gastrointestinal tract. Its high atomic number effectively blocks X-rays, allowing clear visualization of soft tissues.
- Toxicity: Soluble barium compounds (e.g., BaCl₂, BaCO₃, Ba(NO₃)₂) are highly toxic if ingested, interfering with muscle and nerve function. Insoluble BaSO₄ is safe due to its extremely low solubility.