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I

Iodine

Iodine (I) is a reactive nonmetal with atomic number 53 and an atomic mass of 126.9. Its electron configuration is [Kr] 4d10 5s2 5p5, which gives it 7 valence electrons.

Iodine key data

Symbol I
Atomic number 53
Atomic mass (standard atomic weight) 126.9
Molar mass 126.9 g/mol
Protons 53
Neutrons (most abundant isotope, iodine-127) 74
Electrons (neutral atom) 53
Valence electrons 7
Category Reactive nonmetal
Period 5
Group 17
Block p-block
Noble gas notation [Kr] 4d10 5s2 5p5
Full electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5
Electrons per shell 2, 8, 18, 18, 7

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.

Measured properties of iodine

From PubChem, the US National Institutes of Health. Public domain.

Melting point 386.85 K (113.7 °C), 83rd of 103 by melting point
Boiling point 457.55 K (184.4 °C), 81st of 93 by boiling point
Density 4.93 g/cm³, 64th of 96 by density
State at room temperature Solid
Electronegativity 2.66 (Pauling), 7th of 95 by electronegativity
Van der Waals radius 198 pm, 62nd of 99 by atomic radius
First ionisation energy 10.451 eV (1008.4 kJ/mol), 15th of 102 by first ionisation energy
Electron affinity 3.059 eV (295.1 kJ/mol), 4th of 57 by electron affinity
Common oxidation states +7, +5, +1, −1
Discovered 1811

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Iodine electron configuration

The full electron configuration of iodine is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5. With the noble-gas core collapsed it is [Kr] 4d10 5s2 5p5, and the outermost subshell is 5p5.

Bohr model of iodine (I): a nucleus and five shells holding 2, 8, 18, 18 and 7 electrons from the inside out.
Bohr model of iodine (I). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.
Orbital box diagram of iodine (I), [Kr] 4d10 5s2 5p5, with the krypton core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 5p.
Orbital diagram of iodine (I). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one iodine atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

  1. one atom = 126.9 / (6.02214076 × 10²³)
  2. = 2.1072 × 10⁻²² g

Atoms in one gram

  1. atoms per gram = 6.02214076 × 10²³ / 126.9
  2. = 4.7456 × 10²¹ atoms

Iodine on the periodic table

Iodine is in period 5, group 17, in the p-block.

The other elements of group 17 are fluorine, chlorine, bromine, astatine and tennessine.

The rest of period 5

Work with iodine