Caesium (Cs): Bohr Model, Protons, Neutrons & Electrons
Caesium is element number 55, an alkali metal in group 1, period 6. Its atom has 55 protons, 78 neutrons and 55 electrons arranged in 6 shells: 2, 8, 18, 18, 8, 1.
Protons, neutrons and electrons in caesium
The atomic number of caesium is 55, so every caesium atom has 55 protons. A neutral atom has the same number of electrons, 55. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For caesium-133, that is 133 − 55 = 78 neutrons.
How to draw the Bohr model of caesium
- Draw the nucleus and write 55 p⁺ and 78 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 8 electrons on it.
- Draw the sixth shell and put 1 electron on it.
- Check: 2 + 8 + 18 + 18 + 8 + 1 = 55 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, 8, 1) is the one to draw.
Electron configuration
The electron configuration of caesium is [Xe] 6s¹. Its 1 outer electron decides how it reacts: it tends to lose it, forming Cs⁺ ions.
Key facts about caesium
| Symbol | Cs |
|---|---|
| Atomic number | 55 |
| Relative atomic mass | 132.91 |
| Group · period · block | group 1 · period 6 · s-block |
| Type | Alkali Metal |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 18, 8, 1 |
| Electron configuration | [Xe] 6s¹ |
| Valence electrons | 1 |
| Common ion | Cs⁺ |
| Electronegativity (Pauling) | 0.79 |
| First ionisation energy | 3.89 eV (375 kJ/mol) |
| Atomic radius (empirical) | 260 pm |
Where caesium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does caesium have?
Caesium has 55 protons and 55 electrons (atomic number 55). Its most common isotope, caesium-133, has 133 − 55 = 78 neutrons.
What is the electron configuration of caesium?
[Xe] 6s¹. In shells that is 2, 8, 18, 18, 8, 1.
How many valence electrons does caesium have?
1. Caesium is in group 1, and its outer shell holds 1 electron.
Where is caesium on the periodic table?
In group 1, period 6, in the s-block. It is an alkali metal.
