Symbol

Cs

Atomic Number

55

Atomic Mass

132.9054

Family

Group 1 (Ia) Alkali Metal Pronunciation

SEE-zee-um

Overview

Cesium is a member of the alkali family, which consists of elements in Group 1 (IA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The alkalis include lithium, sodium, potassium, rubidium, and francium. Cesium is considered the most active metal. Although in theory francium is more active than cesium, francium is too rare to have any commercial uses.

Cesium was discovered in 1861 by German chemists Robert Bunsen (1811-99) and Gustav Kirchhoff (1824-87). They found the element using a method of analysis they had just invented: spectroscopy. Spectroscopy is the process of analyzing light produced when an element is heated. The light produced is different for every element. The spectrum (plural: spectra) of an element consists of a series of colored lines.

Cesium is...