Generally, the ionization energy of elements increases across the period, as nuclear charge increases and it decreases down the group. The first Ionization Energy of Barium is less than Calcium as Barium is situated in period 6 of the periodic table and Calcium is situated in period 4. This means that: The outermost electron of Barium is far away from the nucleus as compared to Calcium and there is a greater distance in between them. There is a greater number of intervalence shells, and so intervalence electrons in Barium than Calcium, which provide a stronger shielding effect. These two factors outweigh the effect of increased nuclear charge on Barium, so Barium has lesser Ionization Energy than Calcium.