A. Phosphorus has a greater first ionization energy than magnesium, and the reason for this is because P has a greater effective nuclear charge (Zeff) than Mg.
B. The first ionization energy of an atom is smaller than the second ionization energy of the same atom.
C. The following process is an endothermic process and represents the first ionization energy of an atom A: A(g) ? A1+(g) + e1-
D. Fluorine has a greater first ionization energy than iodine, and the reason for this is because fluorine has a greater amount of shielding of outer electrons by inner electrons than does iodine.