Explain why successive ionization energies of an element increase and how they account for the existence of three main energy levels in the sodium atom

Successive ionization energies increase because for the same nuclear charge (positive pull of the nucleus) there are fewer electrons each time one is removed, thus more energy is required to remove successive electrons. The successive ionisation energies of Sodium show large increases in ionization energy when the 2nd and 10th electrons are removed. This shows that the 1st electron is further from the nucleus than the 2nd electron and same with the 9th and 10th electrons. Large increases in IE like this indicate changes in the main energy levels of the atom.