SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Caesium (Cs). Diagram of the nuclear composition and electron configuration of an atom of caesium-133 (atomic number: 55), the most common isotope of this element. The nucleus consists of 55 protons (red) and 78 neutrons (blue). The stability of an element's outer electrons determines its chemical and physical properties. Caesium, used in atomic clocks, is an alkali metal in group 1, period 6, and the s-block of the periodic table. It is a soft, explosively reactive metal that is denser than water, melting at 28 degrees Celsius.
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