Ionization Energy Definition The Ionization Energy may be defined as the amount of energy required to remove an electron from an isolated gaseous atom of an element in its ground state. Ionization Energy is usually determinrd by potential gradient through which elec tron is accelerated, so that the term Ionization Potential is also used to refer the same value. Unit Ionization Energy is measured in electron volts (ev) per atom or kilojoule per m ol (kJmol-1). Successive I.E. The amount of energy required to remove an electron from an isolated gaseous ato m is called first ionization energy (IE1). Similarly, the energy required for the removal of one more electron (from the un ipositive ion produced above) is reffered to as second ionization energy (IE2) a nd stepwise third, fourth etc ionization energies. There can be as many as IEs a s there are electron in the atom. It has been observed that second IE is higher than the first, third IE is higher than the first, third IE is higher than second IE and so on, i.e., IE1