Caesium is element number 55, a metal with the symbol Cs. Its name comes from the Latin for “sky blue.” Today, the second is defined using the caesium atom.
Atomic number 55132.9
CsCaesiumセシウム (seshiumu)
Symbol
Cs
Atomic weight
132.9
Place in the table
Period 6 · Group 1
Atomic weight from the Chemical Society of Japan, “Table of Standard Atomic Weights with Four Significant Figures (2026)”.
A sky-blue line
Caesium was discovered in 1860 from a blue line seen when light is split finely, and it was named for that color. It was one of the first elements found by its lines of light.
The element that defines the second
In 1967, the second was defined as the time taken by 9,192,631,770 cycles of the radiation from the caesium-133 atom (from a transition between two of its energy levels). This definition is still used today.
Caesium is in the sixth row of Group 1, below sodium, potassium and rubidium.
Think about it
Q1What do you think people used long ago to decide how long a second is?
Q2What would it take to count nearly 9.2 billion cycles?
Q3Look for elements found by the color of light in the Room of the Elements.
There is no single right answer. Talking it over with someone at home is a good way to learn.
Did you know? — The nucleus of a caesium-133 atom contains 55 protons. 55 is also caesium’s atomic number.