The second ionization energy refers to the energy required to remove a second electron from a singly charged ion. In the case of third period elements, the order of their second ionization energies can be determined based on their electron configurations and trends in the periodic table.
The electron configurations of the third period elements are as follows:
Na: 1s² 2s² 2p⁶ 3s¹ Mg: 1s² 2s² 2p⁶ 3s² Al: 1s² 2s² 2p⁶ 3s² 3p¹ Si: 1s² 2s² 2p⁶ 3s² 3p² P: 1s² 2s² 2p⁶ 3s² 3p³ S: 1s² 2s² 2p⁶ 3s² 3p⁴ Cl: 1s² 2s² 2p⁶ 3s² 3p⁵ Ar: 1s² 2s² 2p⁶ 3s² 3p⁶
The general trend for ionization energy across a period is that it increases from left to right due to increasing nuclear charge. Based on this trend, we can expect the second ionization energies to generally increase as we move from left to right in the third period.
However, there are a few exceptions in the case of third period elements. In particular, removing a second electron from a p-block element (Group 15 and onwards) requires breaking into a stable half-filled or fully filled subshell, which results in increased stability and higher ionization energy. This phenomenon is known as the "half-filled and fully filled subshell stability."
Considering these trends, the correct order of the second ionization energies of the third period elements is as follows:
Na < Mg < Al < Si < P < S < Cl < Ar
Please note that while this order generally holds, there may be slight variations due to other factors such as electron-electron repulsion and orbital overlap.