HalogenElement 17 · p-block

Chlorine (Cl)

Chlorine (Cl) is a halogen with atomic number 17 and a standard atomic weight of 35.45. At room temperature it is a gas that melts at -101 °C and boils at -34 °C.

17ClChlorine35.45

Atomic number

protons

Atomic weight

35.45

CIAAW 2024

Melting point

-101 °C

171.65 K

Boiling point

-34 °C

239.11 K

Density

3.2 g/L

at 0 °C, 1 atm

Electronegativity

3.16

Pauling scale

State at 25 °C

Gas

Discovered

1774

Sweden

Chlorine at a glance

Identity

Symbol	Cl
Atomic number (Z)	17
Atomic weight	35.45 ± 0.01 (CIAAW 2024)
Category	Halogen
Group	17 — halogens
Period	3
Block	p-block
CAS number	7782-50-5

Atomic properties

Electron configuration	[Ne] 3s² 3p⁵ Full: 1s² 2s² 2p⁶ 3s² 3p⁵
Electrons per shell	2, 8, 7
Oxidation states	-1, +1, +3, +5, +7 Less common: +2, +4, +6
Electronegativity (Pauling)	3.16
First ionisation energy	12.968 eV (1,251.2 kJ/mol) 2nd: 23.814 eV · 3rd: 39.8 eV
Electron affinity	3.613 eV
Atomic radius (empirical)	100 pm
Covalent radius	99 pm
Van der Waals radius	175 pm

Physical properties

State at 25 °C	Gas
Melting point	-101.5 °C · -150.7 °F · 171.65 K
Boiling point	-34.04 °C · -29.3 °F · 239.11 K
Density	3.2 g/L (gas at 0 °C, 1 atm)
Specific heat capacity	0.479 J/(g·K)
Thermal conductivity	0.009 W/(m·K)
Crystal structure	Orthorhombic

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	³⁵Cl 75.8%, ³⁷Cl 24.2%
Stable isotopes	4
Known isotopes	25
Abundance in Earth's crust	145 mg/kg
Abundance in seawater	19,400 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Chlorine electron configuration

Chlorine has 17 electrons. Written in short form, its ground-state configuration is [Ne] 3s² 3p⁵ — the electrons of the noble gas neon, plus 3s² 3p⁵.Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁵.

As a member of group 17 it has 7 valence electrons in its outer shell (shell 3), which largely decide its chemistry.

3s↑↓

3p↑↓↑↓↑

Discovery and name

Discovered

1774 — Carl Wilhelm Scheele (recognised as an element by Humphry Davy, 1810) · Sweden.

Name

Greek chlōros, pale green — the colour of the gas.

Known for

A toxic yellow-green gas. Chlorine disinfects drinking water and pools and is used to make PVC; as chloride it is half of table salt.

Isotopes of chlorine

Naturally occurring chlorine is a mix of 2 isotopes. Each has 17 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
³⁵Cl	17	18	75.8%	stable
³⁷Cl	17	20	24.2%	stable

25 isotopes of chlorine are known in our data set (half-lives as published in nuclear data tables).

Chlorine compared with group 17

The other elements in group 17 (halogens), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Fluorine	9	18.998	-220 °C	-188 °C	1.696 g/L	3.98
Chlorine	17	35.45	-101 °C	-34 °C	3.2 g/L	3.16
Bromine	35	79.904	-7.2 °C	59 °C	3.103 g/cm³	2.96
Iodine	53	126.90	114 °C	184 °C	4.933 g/cm³	2.66
Astatine	85	[210]	302 °C (est.)	—	7 g/cm³ (pred.)	2.2
Tennessine	117	[294]	—	—	7.2 g/cm³ (pred.)	—

Frequently asked questions

6 answers

What is the atomic mass of chlorine?

The standard atomic weight of chlorine is 35.45 ± 0.01 (CIAAW 2024), so one mole weighs about 35.45 grams.

How many protons, neutrons and electrons does chlorine have?

A neutral chlorine atom has 17 protons and 17 electrons. Its most common isotope, chlorine-35, has 18 neutrons.

What is the electron configuration of chlorine?

[Ne] 3s² 3p⁵ (in full: 1s² 2s² 2p⁶ 3s² 3p⁵).

What are the melting and boiling points of chlorine?

Chlorine melts at -101.5 °C (-150.7 °F) and boils at -34.04 °C (-29.3 °F).

Is chlorine a metal?

Chlorine is a nonmetal (a halogen). It is in group 17, period 3, and is a gas at room temperature.

Who discovered chlorine?

1774 — Carl Wilhelm Scheele (recognised as an element by Humphry Davy, 1810) (Sweden).