Chlorine
Chlorine (Cl) is a reactive nonmetal with atomic number 17 and an atomic mass of 35.45. Its electron configuration is [Ne] 3s2 3p5, which gives it 7 valence electrons.
Chlorine key data
| Symbol | Cl |
|---|---|
| Atomic number | 17 |
| Atomic mass (standard atomic weight) | 35.45 |
| Molar mass | 35.45 g/mol |
| Protons | 17 |
| Neutrons (most abundant isotope, chlorine-35) | 18 |
| Electrons (neutral atom) | 17 |
| Valence electrons | 7 |
| Category | Reactive nonmetal |
| Period | 3 |
| Group | 17 |
| Block | p-block |
| Noble gas notation | [Ne] 3s2 3p5 |
| Full electron configuration | 1s2 2s2 2p6 3s2 3p5 |
| Electrons per shell | 2, 8, 7 |
Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.
The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.
Measured properties of chlorine
From PubChem, the US National Institutes of Health. Public domain.
| Melting point | 171.65 K (−101.5 °C), 94th of 103 by melting point |
|---|---|
| Boiling point | 239.11 K (−34.04 °C), 83rd of 93 by boiling point |
| Density | 0.003214 g/cm³, 89th of 96 by density |
| State at room temperature | Gas |
| Electronegativity | 3.16 (Pauling), 3rd of 95 by electronegativity |
| Van der Waals radius | 175 pm, 84th of 99 by atomic radius |
| First ionisation energy | 12.968 eV (1251.2 kJ/mol), 9th of 102 by first ionisation energy |
| Electron affinity | 3.617 eV (349 kJ/mol), 1st of 57 by electron affinity |
| Common oxidation states | +7, +5, +1, −1 |
| Discovered | 1774 |
Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.
Chlorine electron configuration
The full electron configuration of chlorine is 1s2 2s2 2p6 3s2 3p5. With the noble-gas core collapsed it is [Ne] 3s2 3p5, and the outermost subshell is 3p5.
Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.
Mass of one chlorine atom
A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.
Mass of one atom
- one atom = 35.45 / (6.02214076 × 10²³)
- = 5.8866 × 10⁻²³ g
Atoms in one gram
- atoms per gram = 6.02214076 × 10²³ / 35.45
- = 1.6988 × 10²² atoms
Chlorine on the periodic table
Chlorine is in period 3, group 17, in the p-block.
The other elements of group 17 are fluorine, bromine, iodine, astatine and tennessine.