The __________ is the energy required to remove an electron from an atom's outermost shell, and it generally increases across a period due to increasing nuclear charge.

The first ionization energy is the energy required to remove an electron from an atom’s outermost shell, and it generally increases across a period because the nuclear charge pulls electrons more tightly. As you move left to right, the nucleus gains protons while the electrons stay in the same shell, so the attraction between the nucleus and the outer electron grows. This stronger attraction means more energy is needed to detach the electron. For example, sodium’s first ionization energy is about 5. 1 eV, whereas chlorine’s is about 13.