what is ionization energy

Ionization energy is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, positive ion, or molecule. It is expressed in electronvolts (eV) or joules (J) in physics, and in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol) in chemistry. The first ionization energy is the energy required to remove the first electron from a neutral atom, while the second ionization energy is the energy required to remove the second electron, and so on. Ionization energy generally increases from left to right within a period and from bottom to top within a group in the periodic table