Iodine (I): Bohr Model, Protons, Neutrons & Electrons
Iodine is element number 53, a halogen in group 17, period 5. Its atom has 53 protons, 74 neutrons and 53 electrons arranged in 5 shells: 2, 8, 18, 18, 7.
Protons, neutrons and electrons in iodine
The atomic number of iodine is 53, so every iodine atom has 53 protons. A neutral atom has the same number of electrons, 53. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For iodine-127, that is 127 − 53 = 74 neutrons.
How to draw the Bohr model of iodine
- Draw the nucleus and write 53 p⁺ and 74 n⁰ inside it.
- Draw the first shell and put 2 electrons on it (the first shell holds at most 2).
- Draw the second shell and put 8 electrons on it.
- Draw the third shell and put 18 electrons on it.
- Draw the fourth shell and put 18 electrons on it.
- Draw the fifth shell and put 7 electrons on it.
- Check: 2 + 8 + 18 + 18 + 7 = 53 electrons, the same as the number of protons.
From potassium onwards the shells don’t simply fill as 2, 8, 8: the fourth shell starts before the third is full, so the real arrangement above (2, 8, 18, 18, 7) is the one to draw.
Electron configuration
The electron configuration of iodine is [Kr] 5s² 4d¹⁰ 5p⁵. Its 7 outer electrons decide how it reacts: it tends to gain 1 more, forming I⁻ ions.
Key facts about iodine
| Symbol | I |
|---|---|
| Atomic number | 53 |
| Relative atomic mass | 126.9 |
| Group · period · block | group 17 · period 5 · p-block |
| Type | Halogen |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 18, 7 |
| Electron configuration | [Kr] 5s² 4d¹⁰ 5p⁵ |
| Valence electrons | 7 |
| Common ion | I⁻ |
| Electronegativity (Pauling) | 2.66 |
| First ionisation energy | 10.45 eV (1008 kJ/mol) |
| Atomic radius (empirical) | 140 pm |
Where iodine sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does iodine have?
Iodine has 53 protons and 53 electrons (atomic number 53). Its most common isotope, iodine-127, has 127 − 53 = 74 neutrons.
What is the electron configuration of iodine?
[Kr] 5s² 4d¹⁰ 5p⁵. In shells that is 2, 8, 18, 18, 7.
How many valence electrons does iodine have?
7. Iodine is in group 17, and its outer shell holds 7 electrons.
Where is iodine on the periodic table?
In group 17, period 5, in the p-block. It is a halogen.
