Ionization energy increases across a period primarily because the nuclear charge increases while the shielding effect remains roughly constant. As you move from left to right across a period in the periodic table, more protons are added to the nucleus, increasing the positive charge (nuclear charge). This stronger nuclear attraction pulls the outer electrons closer to the nucleus, decreasing the atomic radius and making the electrons more tightly held. Since the electrons are more strongly attracted and held closer to the nucleus, more energy is required to remove an electron, causing ionization energy to increase across a period. Key factors involved are:
- Increased nuclear charge (more protons) pulling electrons more strongly.
- Slightly decreasing atomic radius, so electrons are closer to the nucleus.
- Shielding effect by inner electrons stays nearly the same since electrons are added to the same shell.
- Electrons are held more tightly, so more energy is needed to remove one.
There are minor exceptions due to electron subshell configurations, but the general trend is an increase in ionization energy across a period because of these factors.
