The most reactive element among cesium, barium, and lutetium is cesium. Cesium is located at the extreme left of Period 6 and has the atomic number 55. It has a valence electron configuration of 6s^1. This single valence electron is loosely held by the nucleus making it highly reactive. Cesium readily loses this outermost electron to form positive ions, giving it strong reducing properties. In contrast barium and lutetium are less reactive as they have higher ionization energies and their valence electrons are more tightly bound to the nucleus.