---
title: "Iodine (I): Atomic Mass, Electron Configuration & Properties"
url: https://www.moldraw.com/elements/iodine.html
description: "Iodine (I), element 53: atomic mass 126.9, electron configuration [Kr] 4d¹⁰ 5s² 5p⁵, group 17, period 5, p-block reactive nonmetal. Melting and boiling point, density, electronegativity and shells."
lang: en
publisher: Scidart Academy
generated: 2026-11-03
---
53I126.9

# Iodine (I) – element 53

Iodine is a chemical element with the symbol **I** and atomic number **53**. It is a reactive nonmetal in group 17 (halogens), period 5 of the periodic table (p-block), and is a solid at room temperature.

Iodine (I) has atomic number 53, atomic mass 126.9 u and electron configuration [Kr] 4d10 5s2 5p5.

[Download periodic table PDF](https://www.moldraw.com/printables/periodic-table.pdf)[Open interactive periodic table](https://www.moldraw.com/tools/free-chem-tools/interactive-periodic-table.html)

## Iodine facts

| Symbol | I |
| --- | --- |
| Atomic number | 53 |
| Atomic mass | 126.9 u (g/mol) |
| Category | reactive nonmetal |
| Group | group 17 (halogens) |
| Period | 5 |
| Block | p-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5 |
| Noble-gas notation | [Kr] 4d10 5s2 5p5 |
| Electrons per shell | 2, 8, 18, 18, 7 |
| Valence electrons | 7 |

## Bohr model and shells

Iodine has 53 electrons in 5 shells: 2, 8, 18, 18, 7. Full configuration and noble-gas notation are in the table; work out any element or ion with the [electron configuration calculator](https://www.moldraw.com/tools/free-chem-tools/electron-configuration-calculator.html).

## Physical and atomic properties

| Melting point | 386.85 K (113.7 °C) |
| --- | --- |
| Boiling point | 457.4 K (184.2 °C) |
| Density | 4.933 g/cm³ |
| Electronegativity (Pauling) | 2.66 |
| Electron affinity | 295.153 kJ/mol |
| Ionization energies | 1008.4, 1845.9, 3180 kJ/mol (1st, 2nd, 3rd) |
| Discovered by | Bernard Courtois |

Use the atomic mass in the [molar mass calculator](https://www.moldraw.com/tools/free-chem-tools/molar-mass-calculator.html) for any formula containing I.

## Position in the periodic table

Tap any element to open its page.

## Frequently asked questions

### What is the atomic mass of iodine?

The atomic mass of iodine is 126.9 u, so one mole of iodine atoms weighs 126.9 g.

### What is the electron configuration of iodine?

Iodine has 53 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁵, or [Kr] 4d¹⁰ 5s² 5p⁵ in noble-gas notation. Electrons per shell: 2, 8, 18, 18, 7.

### How many valence electrons does iodine have?

Iodine has 7 valence electrons (group 17).

### Is iodine a metal?

No. Iodine is a nonmetal (reactive nonmetal).

### Where is iodine on the periodic table?

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

Data: [Periodic-Table-JSON](https://github.com/Bowserinator/Periodic-Table-JSON) (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.