The ionization energy is the energy required to remove an electron from an atom or ion in the gaseous state. In the case of magnesium (Mg), it has three electrons in its outermost energy level (valence electrons), which are relatively loosely held compared to the inner electrons.
The first ionization energy refers to the energy required to remove the first valence electron from a neutral atom of magnesium. This electron is relatively easy to remove because it is located in the outermost energy level and experiences a weaker effective nuclear charge compared to the inner electrons. As a result, it takes less energy to overcome the attraction between the single valence electron and the positively charged nucleus.
The second ionization energy is the energy required to remove the second valence electron from a singly ionized magnesium ion (Mg+). After the first electron is removed, the remaining ion becomes positively charged, resulting in a stronger effective nuclear charge on the remaining electrons. Therefore, the second electron requires more energy to be removed since it experiences a stronger attraction to the nucleus.
Now, when it comes to the third ionization energy of magnesium, it is lower than the first and second ionization energies. This is because after the second valence electron is removed, the resulting ion (Mg2+) has a configuration similar to that of a noble gas (neon) with a completely filled outermost energy level. Noble gas configurations are particularly stable due to the filled energy levels, and as a result, it requires a significantly higher amount of energy to remove an electron from a stable, closed-shell configuration. Hence, the third ionization energy of magnesium is higher than the first and second ionization energies.