Final answer:
Across a period on the periodic table, the ionization energy generally increases due to a stronger electrostatic attraction between the valence electrons and an increasingly positively charged nucleus.
Step-by-step explanation:
The trend in ionization energy as you move across a period in the periodic table, according to Coulomb's law, is that ionization energy generally increases. This happens because as we move from left to right across a period, the nuclear charge of the atom (number of protons) increases. This results in a stronger attraction between the nucleus and the valence electrons, making it more difficult to remove an electron. Hence, the increased ionization energy. Additionally, the valence electrons do not screen each other very effectively, allowing the effective nuclear charge to increase, which in turn attracts the valence electrons more strongly and increases the ionization energy.