Strontium (Sr) Study Guide for CBSE, JEE, NEET
Introduction to Strontium (Sr)
Strontium, a silvery-white and highly reactive metal, is an alkaline earth metal. It is never found free in nature but is present in minerals like strontianite (SrCO₃) and celestite (SrSO₄). Its most recognized application is in pyrotechnics, where its compounds impart a vivid crimson-red color to fireworks. The radioactive isotope Strontium-90 (⁹⁰Sr) is a significant fission product in nuclear reactions due to its bone-seeking properties, mimicking calcium.
CBSE/JEE Quick Revision Notes
- Symbol: Sr
- Atomic Number: 38
- Atomic Mass: 87.62 g/mol
- Group: 2 (Alkaline Earth Metals)
- Period: 5
- Valency: +2
- Nature: Soft, silvery-white, ductile, and malleable metal. Highly reactive.
- Oxidation State: +2 (predominant and almost exclusive in its compounds).
- Electronegativity (Pauling scale): 0.95
- Density: 2.64 g/cm³ (at 20 °C)
- Melting Point: 777 °C (1431 °F)
- Boiling Point: 1377 °C (2511 °F)
- Flame Test Color: Crimson red (characteristic of Strontium).
Electron Configuration & Bonding Behavior
Electron Configuration
- Ground State Electron Configuration:
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 5s² - Condensed Electron Configuration:
[Kr] 5s²
Bonding Behavior
Strontium, belonging to Group 2, readily loses its two valence 5s² electrons to achieve a stable noble gas configuration (that of Krypton).
- Ion Formation: It forms a dipositive cation, Sr²⁺.
- Ionization Energies: Both the first and second ionization energies are relatively low, reflecting its metallic character and ease of electron loss.
- Electronegativity: Its low electronegativity indicates a strong tendency to form ionic bonds.
- Bonding Type: Strontium predominantly forms ionic compounds with non-metals (e.g., SrCl₂, SrO), exhibiting minimal covalent character.
Crucial Chemical Reactions
Strontium is a highly reactive metal, reacting readily with various substances.
Reaction with Oxygen (Air)
Strontium tarnishes rapidly in air, forming strontium oxide.
2Sr(s) + O₂(g) → 2SrO(s)
Reaction with Water
Strontium reacts vigorously with cold water to produce strontium hydroxide and hydrogen gas.
Sr(s) + 2H₂O(l) → Sr(OH)₂(aq) + H₂(g)
Reaction with Halogens
Strontium reacts directly with halogens to form ionic halides.
Sr(s) + Cl₂(g) → SrCl₂(s) (Strontium chloride)
General form: Sr(s) + X₂(g) → SrX₂(s) (where X = F, Cl, Br, I)
Reaction with Acids
Strontium reacts readily with dilute acids to produce strontium salt and hydrogen gas.
Sr(s) + 2HCl(aq) → SrCl₂(aq) + H₂(g) (Strontium chloride)
General form: Sr(s) + 2HX(aq) → SrX₂(aq) + H₂(g)
Thermal Decomposition of Strontium Carbonate
Strontium carbonate decomposes at high temperatures to form strontium oxide and carbon dioxide.
SrCO₃(s) → SrO(s) + CO₂(g)
Flame Test
When heated in a flame, strontium compounds produce a characteristic crimson-red color, used for qualitative analysis. This is due to the excitation of valence electrons, which then emit light at specific wavelengths upon returning to their ground state.
Industrial and Biological Importance
Industrial Importance
- Pyrotechnics: Strontium salts, particularly strontium nitrate (Sr(NO₃)₂), are widely used to produce the brilliant crimson-red color in fireworks, signal flares, and tracer ammunition.
- Glass Manufacturing: Strontium carbonate (SrCO₃) is incorporated into the glass of cathode ray tubes (CRTs) in older televisions and monitors to absorb X-rays emitted during operation.
- Ferrite Magnets: Strontium ferrite (SrFe₁₂O₁₉) is a crucial component in the production of permanent magnets used in small motors, loudspeakers, and various electronic devices.
- Electrolytic Production of Zinc: Strontium is used to remove lead impurities during the electrolytic refining of zinc.
Biological Importance
- Bone Metabolism: Due to its chemical similarity to calcium, strontium can be incorporated into the bone matrix.
- Strontium Ranelate: This strontium-containing pharmaceutical is used in the treatment of osteoporosis, a bone-weakening disease. It works by stimulating bone formation and inhibiting bone resorption.
- Radioactive Isotope Sr-90: This is a hazardous radioactive isotope (beta emitter with a half-life of 28.8 years) produced in nuclear fission. It is biologically dangerous because, like calcium, it can be absorbed into bones and teeth, potentially leading to bone cancers and leukemia.