Answer:
Chlorine (Cl)
Step-by-step explanation:
Ionization Energy is defined as, "the minimum energy required to knock out or remove the valence electron from valence shell of an atom".
Trends in Periodic table:
Along Periods:
Ionization Energy increases from left to right along the periods because moving from left to right in the same period the number of protons (atomic number) increases but the number of shells remain constant hence, resulting in strong nuclear interactions and electrons are more attracted to nucleus hence, requires more energy to knock them out.
Along Groups:
Ionization energy decreases from top to bottom along the groups because the number of shells increases and the distance between nucleus and valence electrons also increases along with increase in shielding effect provided by core electrons. Therefore, the valence electrons experience less nuclear attraction and are easily removed.
Conclusion:
As Barium is present down the group and to the left of periodic table, K present at the left of the periodic table, Arsenic present below Phosphorous will have less ionization energies as compared to P. Hence, only Chlorine present at the right extreme of periodic table and right to P will have greater Ionization energy.