Ionization potential increases across a period in the periodic table because atoms become larger and less able to hold onto their outermost electrons.

Ionization potential actually rises across a period because the atoms get smaller, not larger, and the outer electrons are pulled more tightly toward the nucleus. As the atomic number increases, the nuclear charge rises while the shielding effect stays roughly the same, so each new electron enters the same shell but feels a stronger attraction. This tighter grip means more energy is needed to remove an electron, raising the ionization potential. For example, sodium (Na) has a lower ionization potential than magnesium (Mg) even though both are in the same period, because Mg’s extra proton pulls its valence electron harder. Thus, the trend is driven by a stronger nuclear pull on a smaller atom rather than a larger size.