Atomic Number (Z): 55

Number of Protons: 55

Number of Neutrons: 78 (for the most common isotope, Cs-133)

Number of Electrons: 55

Cesium is a highly reactive metal that is part of the alkali metal group on the periodic table. It is a silvery-gold color and is known for its low ionization energy, which means it readily loses an outer electron. Cesium is used in various applications, including atomic clocks, photocells, and as a propellant in ion propulsion systems for spacecraft.