Considering the definition of first and second ionization energy, the second ionization energy of lithium is:
Li⁺ → Li²⁺ + 1 e-
In first place, the electrons are attracted to the nucleus and it is necessary to provide energy to start them. Ionization energy is the energy required to remove an electron from a gaseous atom, isolated and in a ground state. The electrons in the last shell, which are the weakest attracted to the nucleus, are always lost. In this way the neutral atom becomes a gaseous cation (ion with a positive charge).
So, in this case, the first ionization energy of lithium is:
Li → Li⁺ + 1 e-
The energy required to remove a second electron is called the second ionization energy and due to the difficulty of removing an electron from a positive particle, its value is greater than the first ionization energy (the volume of a positive ion is less than that of the neutral atom and the electrostatic force is greater in the positive ion than in the atom)
Finally, in this case, the second ionization energy of lithium is:
Li⁺ → Li²⁺ + 1 e-
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