What is the atomic number of chlorine?

Chlorine has an atomic number of 17, meaning each neutral chlorine atom contains 17 protons in its nucleus.

What is the electron configuration of chlorine?

The full electron configuration is [Ne] 3s2 3p5, or written out: 1s2 2s2 2p6 3s2 3p5. It has 7 valence electrons, one short of a full outer shell.

Is chlorine a gas at room temperature?

Yes. Chlorine is a diatomic gas (Cl2) at standard conditions (25 °C, 1 atm). Its boiling point is −34.05 °C, well below room temperature.

What are the common oxidation states of chlorine?

Chlorine most commonly appears as −1 (chloride ion in salts and solutions), but it also exhibits +1, +3, +5, and +7 in oxoacids and their salts (e.g., hypochlorite, chlorate, perchlorate).

Why is chlorine used in water treatment?

Chlorine is a powerful oxidizing disinfectant that kills bacteria, viruses, and other pathogens at very low concentrations (typically 0.2–4 ppm), making tap water safe to drink.

What is the atomic mass of chlorine?

The standard atomic weight of chlorine is 35.45 u (unified atomic mass units), reflecting the natural abundance of its two stable isotopes: Cl-35 (75.77%) and Cl-37 (24.23%).

Chlorine Element Properties

Explore the complete physical and chemical properties of chlorine (Cl, atomic number 17) — the pale yellow-green halogen used in water treatment and industrial chemistry.

17 Cl 35.45

Chlorine

Halogen — Group 17, Period 3 — discovered 1774 by Carl Wilhelm Scheele

Diatomic gas at STP Nonmetal Strong oxidizer

Property	Value	Notes
Element	Chlorine	Official IUPAC name
Symbol	Cl	Chemical symbol on the periodic table
Atomic Number	17	Number of protons in the nucleus
Atomic Mass	35.45 u	Standard atomic weight (IUPAC 2021)
Group	17 (VIIA)	Halogen group
Period	3	Third row of the periodic table
Block	p-block	Outermost electron in a p orbital
Classification	Nonmetal / Halogen	Chemical family
CAS Number	7782-50-5	Cl2 molecule registry number
Discovery	1774 — Carl Wilhelm Scheele	Isolated as a gas from hydrochloric acid + MnO2
Named By	Humphry Davy, 1810	From Greek "chloros" (pale green)
Natural Occurrence	Never free; as chloride salts (NaCl, KCl)	Highly reactive, always found combined

Property	Value	Notes
State at STP	Gas (Cl2)	Pale yellow-green diatomic gas at 25 °C, 1 atm
Color	Pale yellow-green	Distinctive color; strong pungent odor
Density (gas, STP)	3.214 g/L	About 2.5× heavier than air
Density (liquid, −34 °C)	1.5625 g/cm³	At boiling point
Melting Point	−101.5 °C (−150.7 °F)	171.65 K — solid Cl2 forms below this
Boiling Point	−34.05 °C (−29.27 °F)	239.11 K — evaporates well below room temperature
Critical Temperature	143.8 °C (417 K)	Above this, Cl2 cannot be liquefied by pressure alone
Critical Pressure	7.991 MPa	Pressure at the critical point
Triple Point	−101.0 °C, 1.392 kPa	All three phases coexist
Solubility in Water	7.29 g/L at 20 °C	Forms HCl + HOCl (hypochlorous acid) on dissolution
Molar Mass (Cl2)	70.90 g/mol	Diatomic molecule
Atomic Radius	99 pm (covalent)	175 pm van der Waals radius
Ionic Radius (Cl−)	181 pm	Common chloride ion radius
Bond Length (Cl2)	198.8 pm	Cl–Cl single bond in diatomic molecule

Property	Value	Notes
Electron Configuration	[Ne] 3s² 3p⁵	Core + valence notation (IUPAC)
Full Config	1s² 2s² 2p⁶ 3s² 3p⁵	All 17 electrons written out
Valence Electrons	7	One less than a full octet — highly electronegative
Electronegativity	3.16 (Pauling scale)	Third most electronegative element after F and O
Electron Affinity	348.6 kJ/mol	Energy released when Cl gains one electron to form Cl−
Ionization Energy (1st)	1251.2 kJ/mol	Energy to remove the first electron
Ionization Energy (2nd)	2298 kJ/mol	Removing a second electron is much harder
Common Oxidation States	−1, +1, +3, +5, +7	−1 is most common (chloride ion)
Shell Structure	2, 8, 7	Electrons per shell from innermost
Orbital Diagram (3p)	↑↓ ↑↓ ↑	Three 3p orbitals: two filled, one with one electron
Spectral Lines	837.594 nm, 479.45 nm (visible)	Characteristic emission spectrum
Magnetic Ordering	Diamagnetic	Cl− ion has no unpaired electrons

Property	Value	Notes
Standard Enthalpy of Formation	0 kJ/mol (Cl2 gas)	Reference state for thermochemistry
Bond Dissociation Energy (Cl2)	242.58 kJ/mol	Energy to break the Cl–Cl bond homolytically
Enthalpy of Vaporization	20.41 kJ/mol	At boiling point (−34.05 °C)
Enthalpy of Fusion	6.40 kJ/mol	At melting point (−101.5 °C)
Specific Heat Capacity (gas)	0.479 J/(g·K)	Cp at constant pressure, 25 °C
Thermal Conductivity	0.0089 W/(m·K)	Gas at 25 °C
Standard Molar Entropy (S°)	223.08 J/(mol·K)	Cl2 gas at 298.15 K
Gibbs Free Energy of Formation	0 kJ/mol	Reference state; ΔGf° of Cl2(g) = 0
Heat Capacity Ratio (γ)	1.34	Cp/Cv for diatomic gas
Speed of Sound (gas, 0 °C)	206 m/s	In pure Cl2 at standard conditions
Flash Point	Non-flammable	Cl2 is an oxidizer, not a fuel
Autoignition Temperature	N/A	Does not self-ignite; supports combustion of many materials

Common Uses & Industrial Applications

Water Treatment

Added as Cl2 gas or sodium hypochlorite to municipal water supplies to kill pathogens and prevent waterborne diseases.

PVC Production

Chlorine is a key feedstock for polyvinyl chloride (PVC) — one of the world's most widely used plastics.

Disinfectants & Bleach

Sodium hypochlorite (bleach) and chloramines are used for household disinfection, laundry, and surface sterilization.

Pharmaceuticals

Many drugs contain C–Cl bonds (e.g., antihistamines, antibiotics). Chlorine chemistry is central to medicinal synthesis.

Agrochemicals

Chlorinated pesticides, herbicides, and fungicides (e.g., atrazine, lindane) protect crops worldwide.

Paper & Pulp Bleaching

Chlorine dioxide (ClO2) whitens paper pulp without generating dioxins, replacing elemental Cl2 in modern mills.

Key Isotopes

Isotope	Mass (u)	Natural Abundance	Half-Life	Notes
³⁵Cl	34.9689 u	75.77 %	Stable	Most abundant stable isotope
³⁷Cl	36.9659 u	24.23 %	Stable	Second stable isotope; two naturally occurring
³⁶Cl	35.9683 u	Trace	301,000 years	Cosmogenic; used in groundwater dating
³⁸Cl	37.9680 u	—	37.24 min	Radioactive; beta emitter

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Summary

Chlorine (symbol Cl, atomic number 17) is a halogen in Group 17 of the periodic table. It is a pale yellow-green gas at room temperature with a pungent, distinctive odor. This reference page collects the authoritative physical and chemical properties of chlorine, from its electron configuration and oxidation states to its thermodynamic constants and most common industrial applications.

How it works

Select a property category from the tabs: Overview, Physical, Electronic, or Thermodynamic.
Each card shows the property name, value with units, and a brief explanation.
Use the search box to filter properties by name across all categories.
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Use the periodic-table tile and tabbed property tables to compare physical, electronic, and thermodynamic data.

Use cases

Students looking up chlorine data for chemistry homework or lab reports.
Engineers designing water-treatment or chlorination systems.
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Pharmacists and chemists checking oxidation states for reaction planning.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu