Ionization energy is the energy required to remove an electron.
A. Bromine is in energy level 3 and has 7 valence electrons, with an IE of 1139.859 kJ/mol, therefore it has the highest ionization energy in this set.
B. Chlorine is in energy level 2 and has 7 valence electrons, with an IE of 1251.186 kJ/mol, therefore it has the highest ionization energy in this set.
The closer the valence electrons are to the nucleus, the stronger the attraction between the positive nucleus and the electrons. So, it will require more energy to remove an electron.
