Chemical element · Atomic number 17
Chlorine
Chlorine in the periodic table: atomic number 17, electron configuration, atomic mass, physical data, oxidation states, media credit and visible sources.
Halogen
gaseous
35.45 u
Documented element sampleA securely sealed ampoule containing yellow-green chlorine gas.
Image credit: James St. John
Auto-oriented, limited to 1600 × 1200 pixels and re-encoded as WebP; the subject was not altered.
Atomic classification
Shell occupancy
Chlorine in the Bohr shell model
This shows the electron distribution of the neutral atom in a simplified shell model.
- K · n=1
- 2 electrons
- L · n=2
- 8 electrons
- M · n=3
- 7 electrons
- Electron configuration
- [Ne]3s2 3p5
- Electrons per shell
- 2 · 8 · 7
- Group
- 17
- Period
- 3
- Block
- P
- Element category
- Halogen
Physical and chemical properties
- Atomic mass
- 35.45 u
- Standard state
- gaseous
- Density
- 0.003214 g/cm³
- Melting point
- 171.65 K
- Boiling point
- 239.11 K
- Electronegativity
- 3.16 (Pauling)
- First ionisation energy
- 12.968 eV
- Oxidation states
- +7, +5, +1, -1
- Discovery
- 1774
Safety and periodic classification
Safety
Safe handling cannot be inferred from Chlorine's position in the periodic table alone. Laboratory, classroom and disposal decisions must follow the documentation for the exact material and its safety data sheet.
Position and comparison
Chlorine is in period 3, group 17 and the P block. Its direct neighbours by atomic number are Sulfur and Argon. The recorded Pauling electronegativity is 3.16. Periodic trends are compared only through the separately sourced neighbouring values.
Sources and scope
PubChem attributes element data to sources including IUPAC, NIST and IAEA. Quanta stores the referenced snapshot locally and leaves unknown values unavailable.
Where Chlorine sits among the 118 elements
The table compares the reported values for Chlorine with every other element that carries the same value in the snapshot. A rank is not a judgement, it is orientation: it says how unusual this value is across the periodic table.
| Value | Chlorine | Position in the data set |
|---|---|---|
| Melting point | 171.65 K | Rank 94 of 103 elements with a reported value. Highest: Carbon. |
| Boiling point | 239.11 K | Rank 83 of 93 elements with a reported value. Highest: Rhenium. |
| Density | 0.003 g/cm³ | Rank 89 of 96 elements with a reported value. Highest: Osmium. |
| Electronegativity (Pauling) | 3.16 | Rank 3 of 95 elements with a reported value. Highest: Fluorine. |
| Ionisation energy | 12.968 eV | Rank 9 of 102 elements with a reported value. Highest: Helium. |
All values and ranks come from the PubChem Periodic Table snapshot, retrieved on 2026-07-15. Elements without a reported value are excluded, so the population differs per row.
The neighbours of Chlorine in the periodic table
- Place in the table
- Group 17, period 3, P block. Classified as halogen, gaseous at standard conditions.
- Direct neighbours
- Atomic number 17. Preceded by Sulfur, followed by Argon.
- Same group
- Same number of outer electrons, and therefore similar reactivity to Fluorine, Bromine, Iodine, Astatine, Tennessine.
- Discovery
- Recorded in the snapshot as: 1774. Who, where and under what circumstances is not part of that record, so this page carries the sourced year and no discovery narrative.
- Oxidation states
- +7, +5, +1, -1. They follow from the electron configuration [Ne]3s2 3p5 and indicate which compounds to expect.
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