53126.9IIodine
Iodine (I) — element 53halogen · group 17, period 5
Iodine is a chemical element with the symbol I and atomic number 53. It is a halogen in group 17, period 5 of the periodic table (the p-block), with a standard atomic mass of 126.9 u. At room temperature (25 °C) iodine is a solid.
💡 Known for: Essential for thyroid hormones; used as an antiseptic and added to iodised salt.
# Iodine properties
| Symbol | I |
|---|---|
| Atomic number | 53 |
| Atomic mass | 126.9 u |
| Category | halogen |
| Group | 17 |
| Period | 5 |
| Block | p-block |
| Electron configuration | [Kr] 4d¹⁰ 5s² 5p⁵ |
| Full configuration | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁵ |
| Electrons per shell | 2, 8, 18, 18, 7 |
| Protons / electrons | 53 / 53 |
| State at 25 °C | solid |
| Melting point | 386.85 K (113.7 °C, 236.7 °F) |
| Boiling point | 457.4 K (184.2 °C, 363.6 °F) |
| Density | 4.93 g/cm³ |
| Electronegativity (Pauling) | 2.66 |
| First ionization energy | 10.451 eV |
| Atomic radius | 115 pm |
| Discovered | 1811 |
# Electron configuration of iodine
The electron configuration of iodine is [Kr] 4d¹⁰ 5s² 5p⁵, written out in full as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁵. Its 53 electrons fill 5 shells as 2, 8, 18, 18, 7, so the outermost shell holds 7 electrons. As a group 17 element, iodine has 7 valence electrons.
# Related elements
TeTelluriumprevious elementXeXenonnext elementFFluorinegroup 17ClChlorinegroup 17BrBrominegroup 17AtAstatinegroup 17TsTennessinegroup 17
# Iodine FAQ
What is the atomic number of iodine?
The atomic number of iodine is 53: every iodine atom has 53 protons in its nucleus. Its symbol is I.
What is the atomic mass of iodine?
The standard atomic mass of iodine is 126.9 u.
What is the electron configuration of iodine?
Iodine's electron configuration is [Kr] 4d¹⁰ 5s² 5p⁵ (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁵), with 2, 8, 18, 18, 7 electrons per shell.
Is iodine a metal?
No. Iodine is a halogen, a nonmetal.
What is the melting point of iodine?
Iodine melts at 386.85 K (113.7 °C, 236.7 °F) and boils at 457.4 K (184.2 °C, 363.6 °F).
What is iodine used for / known for?
Essential for thyroid hormones; used as an antiseptic and added to iodised salt.