Energy
The energy needed to remove an electron from an atom is called
electronegative
valence
ejection
ionization

Ionization energy

Explanation:

Think of it this way: ions are elements with positive or negative charge. If we remove an electron, we give the element a positive charge and it becomes a cation (positive ion). Ionization energy is the energy required to do so - the more electronegative the element, the closer it holds its electrons, the harder they are to remove and the more energy you need.