Chlorine (Cl)
Reactive nonmetal
Chlorine is a sharp-smelling, yellow-green gas and a powerful disinfectant. Compounds of chlorine sanitise drinking water and swimming pools worldwide — and, with sodium, it forms ordinary table salt.
Position on the periodic table
Atomic properties
| Atomic number | 17 |
|---|---|
| Atomic mass | 35.45 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p5 |
| Noble gas shorthand | [Ne] 3s2 3p5 |
| Electrons per shell | 2, 8, 7 |
| Valence electrons (outer shell) | 7 |
| Common oxidation states | -1, +1, +3, +5, +7 |
| Electronegativity (Pauling) | 3.16 |
| Covalent radius (approx.) | 102 pm |
| First ionization energy | 1251 kJ/mol |
| Electron affinity | 348.6 kJ/mol |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Density | 3.2 g/L |
| Melting point | -101.55 °C (171.6 K) |
| Boiling point | -34.04 °C (239.11 K) |
| Appearance | pale yellow-green gas |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of chlorine
Chlorine's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p5, usually shortened to [Ne] 3s2 3p5. Its electrons occupy 3 shells (2, 8, 7), placing it in period 3 of the p-block. The 7 outer-shell electrons drive its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of chlorine
- Disinfecting drinking water and pools
- Bleach (sodium hypochlorite)
- PVC plastic production
- Making solvents, medicines and pesticides
Biological role: Essential as chloride — the main negative ion in body fluids and part of stomach acid (HCl).
Occurrence: Never free in nature; enormous quantities exist as chloride in seawater and salt deposits.
Common compounds of chlorine
NaCl
Sodium chloride
HCl
Hydrochloric acid
NaClO
Sodium hypochlorite (bleach)
History and discovery
Discovered: 1774 — Carl Wilhelm Scheele. Name origin: From Greek chloros, meaning yellow-green.
Carl Wilhelm Scheele made chlorine in 1774; Humphry Davy proved it was an element in 1810.
Safety notes
Chlorine gas is toxic. Never mix bleach with acids or ammonia-based cleaners — the mixture releases dangerous gases.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Chlorine
1. What is the chemical symbol of Chlorine?
2. What is the atomic number of Chlorine?
3. Which category does Chlorine belong to?
4. What is the state of Chlorine at room temperature?
5. Which period of the periodic table is Chlorine in?
Chlorine FAQs
What is the atomic number of chlorine?
Chlorine's atomic number is 17 — every chlorine atom has 17 protons in its nucleus.
What is the symbol for chlorine?
Cl. From Greek chloros, meaning yellow-green.
Is chlorine a metal, nonmetal, or metalloid?
Chlorine is classified as a reactive nonmetal.
What state is chlorine at room temperature?
At about 25 °C, chlorine is a gas. It melts at -101.55 °C.
How many valence electrons does chlorine have?
Chlorine has 7 electrons in its outer shell (shell pattern: 2, 8, 7).
What is chlorine used for?
Key uses include: disinfecting drinking water and pools; bleach (sodium hypochlorite); pvc plastic production.
Related elements
Keep working with this element
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