Quick Facts
| Property | Value |
|---|---|
| Symbol | Cl |
| Atomic Number | 17 |
| Atomic Mass | 35.45 u |
| Category | Halogen |
| Period | 3 |
| Group | 17 |
| Block | p |
| Electron Configuration | [Ne] 3s² 3p⁵ |
Physical Properties
| Property | Value |
|---|---|
| State at 20°C | Gas |
| Density | 0.003214 g/cm³ |
| Melting Point | -101.5°C (171.7 K) |
| Boiling Point | -34.04°C (239.11 K) |
| Appearance | Yellow-green gas with pungent odor |
Atomic Properties
| Property | Value |
|---|---|
| Electron Configuration | [Ne] 3s² 3p⁵ |
| Electronegativity | 3.16 (Pauling scale) |
| First Ionization Energy | 1251.2 kJ/mol |
| Atomic Radius | 99 pm |
| Covalent Radius | 102 pm |
| Van der Waals Radius | 175 pm |
History and Discovery
Discovered by: Carl Wilhelm Scheele Year of Discovery: 1774 Location: Uppsala, Sweden
Etymology
The name "chlorine" comes from the Greek "chloros" meaning "pale green" or "yellow-green," describing the color of the gas.
Discovery Story
Carl Wilhelm Scheele produced chlorine in 1774 by reacting hydrochloric acid with manganese dioxide. He believed it was a compound containing oxygen and called it "dephlogisticated muriatic acid." Humphry Davy proved it was an element in 1810 and gave it the name chlorine based on its color.
Isotopes
| Isotope | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|
| ³⁵Cl | 75.76% | Stable | - |
| ³⁷Cl | 24.24% | Stable | - |
| ³⁶Cl | Trace | 301,000 years | β⁻ |
Occurrence
Natural Abundance
Chlorine is the 21st most abundant element in Earth's crust (about 145 ppm). It is not found free in nature due to its high reactivity. The ocean contains about 1.9% chloride ions. Major sources are rock salt (halite, NaCl) and evaporite deposits.
Extraction and Production
- Chlor-Alkali Process: Electrolysis of brine (sodium chloride solution)
- Byproducts: Sodium hydroxide (NaOH) and hydrogen gas
- Sources: Sea salt, rock salt, brine wells
- Global production: about 70 million tons annually
Applications and Uses
Water Treatment
- Disinfection of drinking water
- Swimming pool sanitation
- Wastewater treatment
- Preventing waterborne diseases
Chemical Industry
- PVC (polyvinyl chloride) production
- Solvent manufacturing
- Bleaching agents
- Hydrochloric acid production
Industrial Applications
- Paper and pulp bleaching
- Textile bleaching
- Metal processing
- Pesticide manufacturing
Other Uses
- Disinfectants and cleaning products
- Pharmaceutical manufacturing
- Refrigerants (declining due to environmental concerns)
Biological Role
Chlorine as chloride ion is essential for life. It is the main anion in extracellular fluid and plays crucial roles in digestion and cellular function.
In the Human Body
- About 0.15% of body mass (95 g in adult)
- Main extracellular anion
- Component of hydrochloric acid in stomach
- Essential for fluid balance and nerve function
- Daily requirement: 2.3 g (as chloride)
Dietary Sources
Table salt (sodium chloride) is the primary source. Also found in seaweed, tomatoes, lettuce, and celery.
Safety and Hazards
Toxicity
Chlorine gas is highly toxic. It was used as a chemical weapon in World War I. Exposure causes severe respiratory damage, burns, and can be fatal.
Handling Precautions
- Strong oxidizer—keep away from reducing agents
- Corrosive to skin, eyes, and respiratory tract
- Immediately dangerous to life at 25 ppm
- Use proper respiratory protection
- Store in well-ventilated areas
Environmental Impact
Chlorine compounds like CFCs caused ozone depletion (now banned). Chlorinated organic compounds can persist in the environment. Proper treatment of chlorine-containing waste is essential.
Interesting Facts
- Chlorine was the first poison gas used in warfare (Ypres, Belgium, 1915)
- The distinctive smell of swimming pools is not chlorine but chloramines formed when chlorine reacts with organic matter
- Adding chlorine to drinking water has been called one of the greatest public health advances
- Chlorine is the third most abundant element dissolved in seawater
- Household bleach is typically a 3-8% solution of sodium hypochlorite (NaClO)