What are the first and second ionization energies?
Ionization Energies are defined as the amount of energy needed to remove an electron from the outer shells of an atom when the atom is in a gaseous state.
The first ionization energy is the amount of energy needed to remove one electron from the outer shell. In chemistry the unit is in kiloJoules or kilocalories per mole.
Generally, the ionization energy for the second, third, forth, etc electrons are greater since it involves removing electrons from an orbital closer to the nucleus. Electrons in closer orbitals have a greater electrostatic attraction for the nucleus so their removal requires more and more energy.
An example would be Chlorine whose first ionization energy in kJ/mol is 1,256, it's second is 2,295, and it's third is 3,850.