Chlorine (Cl)

Chlorinating municipal water is plausibly the single public health measure that has saved the most lives, having largely ended cholera and typhoid in the cities that adopted it from the 1900s onward. The same element was the first chemical weapon used at scale, released from cylinders at Ypres in 1915. Chlorine is a yellow-green gas two and a half times denser than air, which is why it pools in trenches, basements and the deep end of a badly ventilated pool room.

Properties

Atomic number

17

Symbol

Cl

Standard atomic weight

35.45 u

Category

Halogen

Group

17

Period

3

Block

p-block

Electron configuration

[Ne] 3s² 3p⁵

Electrons per shell

2, 8, 7

Melting point

171.65 K — -101.5 °C — -150.7 °F

Boiling point

239.11 K — -34.0 °C — -29.3 °F

Density

0.003214 g/cm³

Electronegativity

3.16 (Pauling)

Oxidation states

-1, +1, +5, +7

Discovered

1774 by Carl Wilhelm Scheele

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁵

Noble gas shorthand

[Ne] 3s² 3p⁵

Shells

K: 2 · L: 8 · M: 7

What it is used for

• Drinking water and swimming pool disinfection
• PVC, which is about a third of all chlorine production
• Household bleach, as sodium hypochlorite
• Solvents, refrigerants and pharmaceutical intermediates
• Titanium dioxide pigment manufacture

Hazards

• Toxic gas; damages the lungs at concentrations around 30 ppm and can kill near 1,000 ppm
• Mixing bleach with acid or with ammonia releases chlorine or chloramine gas
• Denser than air, so it collects in low, enclosed spaces

IUPAC publishes an interval, [35.446, 35.457]; 35.45 is the conventional value. Density is the gas at 0 °C and 101.325 kPa.

Neighbours in the table

Group 17

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Chlorine sits in group 17 with fluorine, bromine, iodine, astatine, tennessine.