Element 17 · Gas at room temperature
Chlorine
Looks like: pale yellow-green gas.
Chlorine is atom number 17 — every Chlorine atom has exactly 17 protons in its center, and that number is what makes it Chlorine and nothing else. Its symbol is Cl. At room temperature it is a gas.
Chlorine is a chemical element with symbol Cl and atomic number 17. It also has a relative atomic mass of 35.5. Chlorine is in the halogen group (17) and is the second lightest halogen following fluorine.
Summary from the source dataset, derived from Wikipedia (CC BY-SA 3.0) · original article
Chlorine sits in period 3, group 17 of the p-block. Its 17 electrons fill shells of 2, 8, 7 — written as [Ne] 3s2 3p5. Its electronegativity is 3.16 on the Pauling scale, and pulling the first electron off takes 1251.2 kJ/mol. The outermost shell holds 7 electrons — the ones that do all the chemistry.
Full configuration: 1s2 2s2 2p6 3s2 3p5
Ionization energies (kJ/mol): 1251.2, 2298, 3822, 5158.6, 6542, 9362, 11018, 33604 … (17 published)
Electron affinity: 348.575 kJ/mol · Electronegativity (Pauling):
3.16
Density: 3.2
g/L · melts 171.6 K · boils 239.11 K
Values exactly as published — see where the data comes from.
Atomic mass
35.45u
Density
3.2g/L
Melts
171.6 K
Boils
239.11 K
The units on this page — explained by Unitii
Chlorine's family
Elements in the same column share their outer-electron count, so they behave alike.
What it becomes
Everyday compounds in our curated set that contain Chlorine — the upward direction of the map: atom → molecule → stuff you know.
Source
Discovered by Carl Wilhelm Scheele.
Periodic-Table-JSON (Bowserinator and contributors); element summaries derived from Wikipedia · CC BY-SA 3.0 · snapshot retrieved 2026-08-21