Chemical element · reference page

17Cl

Chlorine

Chlorine is element 17 (Cl), located in period 3, group 17 and the p block. At room temperature its reference phase is gas. Its electron configuration is [Ne] 3s2 3p5.

Atomic number17

Standard atomic weight[35.446, 35.457]

Group17

Period3

Blockp

Electron configuration[Ne] 3s2 3p5

Electronic structure

What controls its chemistry

Halogens have seven valence electrons. Gaining one electron completes the outer shell, so −1 halide ions are common, while covalent halogen compounds cover a wide range of oxidation states for the heavier members.

The free elements are reactive and are generally not found uncombined in nature. Reactivity and physical state change down the group as atoms become larger and more polarizable.

Chemical forms

Compounds, ions and materials

The free element is molecular or otherwise highly reactive, while halide ions are among the most common chemical forms. Hydrogen halides, metal halides, interhalogen compounds and, for the heavier halogens, oxygen-containing compounds broaden the chemistry well beyond the simple −1 state.

Physical behaviour

State and phase changes

The reference phase at room temperature is gas. The listed density is 3.2 g/L. The melting point is 172.2 K (-100.9 °C). The boiling point is 239.1 K (-34.04 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where chlorine is found and used

In practice, chlorine is encountered in water treatment, pVC and chemical synthesis.

In practice

Examples of real uses

Water treatment

Chlorine is used in disinfection, purification, oxidation or treatment chemistry.

PVC

Chlorine chemistry is used to make polyvinyl chloride, a large-volume plastic for pipes, cable insulation, profiles and many building products.

Chemical synthesis

Chlorine is used as an element, reagent, catalyst component or precursor in the manufacture of other chemicals.

Periodic position

What its neighbours tell us

Chlorine is in period 3 and group 17. In the neighbourhood shown here, it lies between sulfur and argon in the same period; fluorine is above it and bromine below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationdiatomic nonmetal

Phase at room temperatureGas

Electron shells2 · 8 · 7

Density3.2 g/L

Melting point172.2 K (-100.9 °C)

Boiling point239.1 K (-34.04 °C)

Pauling electronegativity3.16

First ionisation energy1 251 kJ/mol

Electron affinity348.6 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of chlorine is associated with Carl Wilhelm Scheele. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is given as the interval [35.446, 35.457]. Natural samples can differ slightly in isotope proportions, so a single universal decimal value would hide real variation between materials.

Chlorine has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The free halogen or reactive compounds of chlorine can be corrosive or toxic. Risk depends strongly on the chemical form and concentration.