Final answer:
Aluminum (Al) is expected to have the largest 3rd ionization energy due to its higher number of valence electrons.
Step-by-step explanation:
The atom with the largest 3rd ionization energy would be Aluminum (Al).
Ionization energy is the energy required to remove an electron from an atom. It increases across a period from left to right in the periodic table because the effective nuclear charge increases, making it more difficult to remove an electron.
Aluminum has a higher ionization energy compared to Sodium (Na), Magnesium (Mg), Silicon (Si), and Phosphorus (P) because it has 13 electrons and 3 valence electrons, which are closer to the nucleus, leading to stronger attraction and higher ionization energy.