Final answer:
Ionization energy is the amount of energy required to remove an electron from an atom in the gas phase. It is always positive and larger values indicate tighter binding of electrons to the atom.
Step-by-step explanation:
Ionization energy (IE) is the amount of energy required to remove an electron from an atom in the gas phase. It is the minimum amount of energy needed to remove an electron from a gaseous atom in its ground state. Ionization energy is always positive and larger values indicate that the electron is more tightly bound to the atom. Some typical units for ionization energies are kilojoules/mole (kJ/mol) or electron volts (eV).