Figure 1. Cesium, with atomic number 55 and atomic weight of 132.905.[1]

Cesium is the 55th element on the periodic table of elements. Cesium is the standard for atomic clocks, and actually provides the definition for the second! Some of its properties are listed below:[2]

Atomic weight 132.90545196
Density (at 20°C) 1.9 g/cm3
Boiling point 944 K
Melting point 301.59 K

Cesium defining our unit of time

Our most common unit of time, the second, was originally defined based on the Earth's rotation; the Earth rotates around its axis once per day, with the second being defined as 1/86,400 of a (mean solar) day.[3] Aside from the fact that there are different definitions for what a "day" is, using the Earth's rotation as a definition for time is not practical due to irregularities in the frequency of its rotation.

Nowadays, Cesium-133 is used as the definition for the second due to the reliable frequency of microwave it emits. The definition is:

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.[3]

In simpler terms, radiant energy, or light, in the form of microwaves is emitted at a very precise frequency from this particular atomic transition. Upon observing exactly 9,192,631,770 periods (or cycles) of this electromagnetic wave, a second is said to have passed.

Note that this frequency occurs when the Cesium-133 atom is in isolation at absolute zero temperature; in reality corrections for this are necessary.[3] Still, the atomic clocks using Cesium keep time within an accuracy of 2 to 3 parts in 1014: this means it may only lose a second once every 1.4 million years![4]

To learn more about the science of Cesium clocks, visit Hyperphysics.


There are many immediately practical uses for extremely accurate clocks. Perhaps the most important is in Earth's Global Positioning System (GPS), which require atomic clocks to provide accurate measurements of distance. They also provide a reliable time reference in scientific research applications, such as measuring variations in the frequencies of pulsars. Cesium clocks have also provided the accuracy necessary to test General Relativity, since an atomic clock operating high above the Earth's surface in a satellite will tick faster than one operating on the ground.[5]


The video below is from the University of Nottingham's periodic videos project.[6] They have created a complete suite of short videos on every element on the periodic table of elements.


  1. Made internally by a member of the Energy Education team, with information from It's Elemental (Jefferson Lab), Available: https://education.jlab.org/itselemental/ele055.html
  2. Jefferson Lab, The Element Cesium [Online], Available: https://education.jlab.org/itselemental/ele055.html
  3. 3.0 3.1 3.2 Bureau International des Poids et Mesures (BIPM). "Unit of time (second)", SI Brochure, Sec. [Online], Available: https://www.bipm.org/en/publications/si-brochure/second.html
  4. United States Naval Observatory. "Cesium Atoms at Work", Time Service Department [Online], Available: http://tycho.usno.navy.mil/cesium.html
  5. Hyperphysics. "Atomic Clocks" [Online], Available: http://hyperphysics.phy-astr.gsu.edu/hbase/acloc.html
  6. See more videos from the University of Nottingham on different elements here: http://www.periodicvideos.com/