Chemical element · reference page

53I

Iodine

Iodine is element 53 (I), located in period 5, group 17 and the p block. At room temperature its reference phase is solid. Its electron configuration is [Kr] 4d10 5s2 5p5.

Atomic number53

Standard atomic weight126.90447(3)

Group17

Period5

Blockp

Electron configuration[Kr] 4d10 5s2 5p5

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 solid. The listed density is 4.933 g/cm³. The melting point is 386.8 K (113.7 °C). The boiling point is 457.4 K (184.2 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where iodine is found and used

In practice, iodine is encountered in thyroid function, nutrition, antiseptics and photography.

In practice

Examples of real uses

Thyroid function

Iodine is required to make thyroid hormones, so both deficiency and excessive intake can disturb thyroid function.

Nutrition

Iodine is relevant to human or animal nutrition in a specific ionic or molecular form.

Antiseptics

Iodine-containing preparations are used for skin disinfection because iodine chemistry can inactivate many microorganisms.

Photography

Historically or currently used in light-sensitive compounds and photographic chemistry.

Periodic position

What its neighbours tell us

Iodine is in period 5 and group 17. In the neighbourhood shown here, it lies between tellurium and xenon in the same period; bromine is above it and astatine 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 temperatureSolid

Electron shells2 · 8 · 18 · 18 · 7

Density4.933 g/cm³

Melting point386.8 K (113.7 °C)

Boiling point457.4 K (184.2 °C)

Pauling electronegativity2.66

First ionisation energy1 008 kJ/mol

Electron affinity295.2 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 iodine is associated with Bernard Courtois. 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 126.90447(3). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Iodine 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 iodine can be corrosive or toxic. Risk depends strongly on the chemical form and concentration.