Cesium (Cs)
Cesium is element 55, an alkali metal in group 1, period 6. A neutral atom has 55 protons, 78 neutrons and 55 electrons, 1 of them valence electrons.
Alkali metal- Atomic number
- 55
- Atomic mass
- 132.91 u
- Valence electrons
- 1
- Protons · neutrons
- 55 · 78
- Electrons per shell
- 2, 8, 18, 18, 8, 1
- At room temperature
- solid
Bohr model of cesium
The Bohr model draws cesium as a nucleus of 55 protons and 78 neutrons, with its 55 electrons on 6 circular shells: 2 in shell 1, 8 in shell 2, 18 in shell 3, 18 in shell 4, 8 in shell 5, 1 in shell 6. Press Build the atom to watch the shells fill from the inside out. The outer ring (the P shell) is dashed: the electrons there are the ones that take part in chemistry.
Real electrons don't travel in neat circles. They live in fuzzy orbitals, shown below, but the Bohr picture is the quickest way to count shells and valence electrons.
Electron configuration of cesium
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹
Short form (noble gas notation): [Xe] 6s¹. The [Xe] stands for the electrons of xenon, the noble gas before it.
How many valence electrons does cesium have?
Cesium has 1 valence electron.
Cesium is in group 1, so it has 1 valence electron. It tends to lose that electron and form a +1 ion.
Protons, neutrons and electrons
| Protons | 55 | equal to the atomic number |
|---|---|---|
| Electrons | 55 | equal to the protons in a neutral atom |
| Neutrons | 78 | mass number 133 − 55 protons |
Atoms of the same element can have different numbers of neutrons (isotopes). The count above uses the rounded atomic mass, the way it's taught in class.
Cesium properties
| Symbol | Cs |
|---|---|
| Category | Alkali metal |
| Group · period · block | 1 · 6 · s |
| Atomic mass | 132.91 u |
| State at 25 °C | solid |
| Melting point | 301.6 K (28.4 °C, 83.2 °F) |
| Boiling point | 944 K (671 °C, 1,240 °F) |
| Density | 1.93 g/cm³ |
| Electronegativity (Pauling) | 0.79 |
| Atomic radius | 343 pm |
| First ionization energy | 3.894 eV |
| Electron affinity | 0.472 eV |
| Oxidation states | +1 |
| Common ion charge | +1 |
| Discovered | 1860 by Robert Bunsen |
Where cesium sits on the periodic table
Group 1: Hydrogen, Lithium, Sodium, Potassium, Rubidium, Francium.
Questions about cesium
How many valence electrons does cesium have?
Cesium has 1 valence electron.
How many protons, neutrons and electrons does cesium have?
A neutral cesium atom has 55 protons and 55 electrons. Its most common form has about 78 neutrons (mass number 133 − 55 protons).
What is the electron configuration of cesium?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹, written in short form as [Xe] 6s¹.
What does the Bohr model of cesium look like?
A nucleus of 55 protons and 78 neutrons, circled by 6 electron shells holding 2, 8, 18, 18, 8, 1 electrons.
What is the atomic number of cesium?
55. The atomic number is the number of protons in the nucleus, and it is what makes an atom cesium.