The atomic number for cesium is 55. This means that an atom of cesium contains 55 protons in its nucleus. The atomic number is a fundamental property of an element, as it defines the identity of the element and its position in the periodic table.
Cesium is a highly reactive alkali metal and is located in group 1 of the periodic table. The atomic number not only identifies the element but also determines its electronic configuration and the way it interacts with other elements. In the case of cesium, it has a single electron in its outermost shell, which plays a significant role in its reactivity.