Ionization energy is an elemental property defined as the amount of energy it takes to remove the outermost electron of a gaseous atom. Cesium has the lowest ionization energy of all the elements, as it takes the least energy to remove the outermost electron of a Cesium atom.
If an element has a low ionization energy, little energy is required to remove the outermost electron of the element. The opposite is true for an element with a high ionization energy. Generally, ionization energy increases on the periodic table as someone moves up and to the right. Helium has the highest ionization energy.