First ionization energy: The first ionization energy is the energy required to remove one electron from a neutral atom. It is a measure of the strength of the electrostatic force between the nucleus and the outermost electrons. The first ionization energy generally increases across a period (row) of the periodic table from left to right, as the number of protons in the nucleus increases and the electrons are more strongly attracted to the nucleus.

Second ionization energy: The second ionization energy is the energy required to remove a second electron from an atom that has already lost one electron. It is generally higher than the first ionization energy because the remaining electrons are more strongly attracted to the nucleus after one electron has been removed.

Third ionization energy: The third ionization energy is the energy required to remove a third electron from an atom that has already lost two electrons. It is generally even higher than the second ionization energy for the same reasons.

In general, the ionization energy of an element increases as more electrons are removed from the atom. This is because the remaining electrons are more strongly attracted to the nucleus, and it requires more energy to overcome this attraction and remove an electron.