The species which will require the least energy for the removal of one
electron is Na+.
What is Ionization energy?
This is the energy required to remove an electron from an isolated neutral
atom. Elements which have lesser electrons in their outermost shell
require a lower ionization energy.
In this scenario, Magnesium is further to the right on the periodic table
than Sodium. This means it will be harder to remove an electron from its
outer shell. This thereby means Na+ will require the least energy for the
removal of one electron.
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