Chlorine
Element 17 · Halogen
Chlorine is the most abundant halogen, and the most familiar: its compounds disinfect pools and tap water worldwide. As a pale green gas it is toxic — it was infamously used as a weapon in World War I — but controlled doses of chlorine compounds make modern sanitation possible. Chlorine's hunger for a single electron makes it a fierce oxidizer and bleaching agent, and the chloride ion it forms is essential to digestion and nerve function. Industry converts it into PVC plastic, solvents, and hydrochloric acid, while table salt pairs it with sodium.
Key facts
| Atomic number | 17 |
|---|---|
| Atomic weight | 35.45 |
| Category | Halogen |
| Group / Period | 17 / 3 |
| Block | p |
| Electron configuration | 1s2 2s2 2p6 3s2 3p5 |
| Common oxidation states | +7, +5, +3, +1, -1 |
| State at STP | Gas |
| Electronegativity | 3.16 |
| Atomic radius | 79 pm |
| Density | 0.00321 g/cm³ |
| Melting point | -101.5 °C |
| Boiling point | -34.04 °C |
Occurrence
Chlorine never occurs free in nature; it sits dissolved in seawater as chloride and in vast rock-salt deposits. Your stomach acid is dilute hydrochloric acid — chlorine at work in digestion.
Applications
Drinking-water disinfection
Bleach (sodium hypochlorite)
PVC plastic production
Table salt (NaCl)
Hydrochloric acid
Common compounds
- Sodium chloride (NaCl)
- Hydrochloric acid (HCl)
- Bleach (NaOCl)
- Polyvinyl chloride (PVC)
The chlorine in a swimming pool and the salt on fries are the same element in two wildly different chemical costumes.
Data source: IUPAC standard atomic weights 2021