Who discovered ionization energy?

Hint: The measure of the difficulty in removing an electron from an atom or ion or the tendency of an atom or ion to surrender an electron is known as ionization energy. Generally, a chemical species loses an electron in the ground state.

Complete answer:
A Russian chemist, Dmitri Mendeleev discovered ionization energy. Ionization energy is the measure of the strength by which an electron is held in a place. It is usually an endothermic process. It gives us an idea of the reactivity of chemical compounds. It is also used to determine the strength of chemical bonds. A high ionization energy indicates that removing an electron from that species is more difficult. If the nucleus of an atom is positively charged, the electrons will be strongly attracted to the nucleus. If an electron lies near the nucleus, then the attraction will be greater than the one when the electron is far from the nucleus. First ionization energy is the energy that is needed to remove the first electron from a neutral atom. Second ionization energy is the energy that is needed to remove the second electron from a unipositive ion.

Note:
If there are more electrons between the outer level and the nucleus, the attraction forces will be less. When there are two electrons in the same orbital, they experience some form of repulsion which creates disturbances in the attraction of the nucleus. Ionization energy will be less in paired electrons as they can be removed easily.