Answer;
Cesium
Step-by-step explanation;
-Cesium atoms have 55 electrons and the shell structure is 2.8.18.18.8.1. The ground state electron configuration of ground state gaseous neutral cesium is [Xe].6s1. The atom has a valence of 1, meaning it requires to loose one electron to form a stable configuration. Compared with alkali metals lithium rubidium and sodium its atom has the greatest amount of energy at ground state.