Iodine at a glance
- Atomic number
- 53
- Symbol
- I
- Atomic mass
- 126.9 u
- Category
- Halogen
- Group / period
- 17 / 5
- Electron configuration
- [Kr] 4d 10 5s 2 5p 5
- State at 25 °C
- Solid
- Melting point
- 113.7 °C
- Boiling point
- 184.3 °C
- Density
- 4.93 g/cm³
- Electronegativity
- 2.66
- 1st ionization energy
- 1008 kJ/mol
- Oxidation states
- −1, +1, +5, +7
- Discovered
- 1811
Atomic structure
A neutral iodine atom has 53 protons and 53 electrons, and its most common isotope carries about 74 neutrons. One atom weighs about 2.11 × 10⁻²⁵ kg, so a mole of iodine (6.022 × 10²³ atoms) weighs 126.9 g. That nucleus holds 1.4 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 18, O 7), which gives the configuration [Kr] 4d 10 5s 2 5p 5. With 7 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has only one stable isotope, I-127. No other element has a stable isotope with mass number 127, so I-127 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) iodine is a solid. It melts at 113.7 °C and boils at 184.3 °C, so it stays liquid across a 70.55-degree range. It stays solid in boiling water but would melt in a hot kitchen oven. Its density is 4.93 g/cm³, 4.93 times as dense as water. Its atoms have a radius of about 140 pm (Tellurium next door is 140 pm). In the solid, its atoms pack into an orthorhombic lattice.
Chemical behaviour
Its electronegativity is 2.66 on the Pauling scale. Pulling off its outermost electron takes 1008 kJ/mol, less than Bromine above it (1140 kJ/mol). A bond between iodine and chlorine would be polar covalent, with an electronegativity difference of 0.5. It releases 295 kJ/mol when it gains an extra electron. Its common oxidation states are −1, +1, +5, +7, so in compounds you will usually meet it as I⁻, I⁺ and I⁵⁺. Group 17 also holds Fluorine, Chlorine, Bromine, Astatine and Tennessine, which share its outer-shell pattern. Its Lewis symbol is I surrounded by 7 dots.
Where it comes from
It makes up 0.45 mg/kg of Earth's crust, the 63rd most abundant element there. In the human body it accounts for about 0.00002 % of body mass. It was first identified in 1811, making it the 47th element to be discovered (counting the 11 known since antiquity together).