Atomic Number
55
Element 55
Cesium (Cs)
Cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C, which makes it one of only five elemental metals that are liquid at or near room temperature.
Atomic Mass
132.91 u
Category
Alkali Metal
Period
6
Group
Group 1: alkali metals and hydrogen
Phase
solid
Density
1.93 g/cm3
Electron Shells
2, 8, 18, 18, 8, 1
Atomic Structure
Protons: 55. Electrons: 55. Neutrons: Varies by isotope.
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹
Why Cesium Matters
Cesium helps connect atomic number 55, alkali metal behavior, period 6 trends, and electron shell structure. Its solid phase, atomic mass of 132.91 u, and electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹ make it useful for comparing bonding patterns across the periodic table.
Discovery and Occurrence
Cesium was discovered 1860 by Robert Bunsen, Gustav Kirchhoff. Cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C, which makes it one of only five elemental metals that are liquid at or near room temperature.
Safety and Handling Context
Cesium is in a highly reactive family. Pure alkali metals can react strongly with water, so demonstrations belong in controlled lab settings.
Related Elements
Frequently Asked Questions
- What is the atomic number of Cesium?
- Cesium has atomic number 55, which means a neutral atom has 55 protons and 55 electrons.
- What is the chemical symbol for Cesium?
- The chemical symbol for Cesium is Cs.
- What category is Cesium in?
- Cesium is classified as a alkali metal and sits in period 6 of the periodic table.
Explore Cesium in 3D
Use the interactive table above to rotate atomic models, compare periodic trends, and move from Cesium to neighboring elements without losing your place.