Chlorine (Cl)
Element 17 · Halogen · Gas at 25 °C · Group 17, Period 3
Chlorine disinfects most of the world's drinking water and swimming pools, and is half of table salt and a building block of PVC plastic.
On the periodic table, chlorine sits in group 17 — the halogens — in period 3. It is one of 6 elements classified as a halogen, and at room temperature (25 °C) it is a gas — one of 11 elements that are gases at room temperature.
The halogens of group 17 are one electron short of a full outer shell, which makes them ferociously good at grabbing electrons from other elements. Their name means “salt-former”: they combine with metals to make salts such as sodium chloride. They exist as diatomic molecules, and the group is the only one to span gas, liquid, and solid at room temperature. Reactivity decreases down the group.
Properties of chlorine
| Atomic number (Z) | 17 |
|---|---|
| Symbol | Cl |
| Atomic mass | 35.45 |
| Category | Halogen |
| Group | 17 |
| Period | 3 |
| Block | p-block |
| Metal / nonmetal | Nonmetal |
| State at 25 °C | Gas |
| Electron configuration | [Ne] 3s2 3p5 |
| Electronegativity (Pauling) | 3.16 |
| Melting point | -101.5 °C (171.65 K / -150.7 °F) |
| Boiling point | -34.04 °C (239.11 K / -29.27 °F) |
| Density | 0.003214 g/cm³ |
Position in the periodic table
Chlorine shares group 17 — the halogens with Fluorine (F), Bromine (Br), Iodine (I), Astatine (At), Tennessine (Ts). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.
Within this dataset, it ranks #3 in electronegativity out of 99 elements with a pauling value.
Physical properties
Chlorine melts at -101.5 °C (171.65 K / -150.7 °F) and boils at -34.04 °C (239.11 K / -29.27 °F), giving it a liquid range of about 67.5 °C at standard pressure.
As a gas at standard conditions, its density is 0.003214 g/cm³ — measured at STP like the other gaseous elements here.
See also: the interactive periodic table, how to read the periodic table, the element families tour.