What trend in ionisation energy occurs as you move across a period?

As you move across a period in the periodic table, ionisation energy typically increases due to the increase in nuclear charge. This happens because, as additional protons are added to the nucleus of the atom, the positive charge increases, which enhances the attractive force between the nucleus and the electrons. Although electrons are being added to the same principal energy level, they do not significantly shield each other from the increasing nuclear charge.

This greater attraction means that more energy is required to remove an electron from the outer shell, resulting in higher ionisation energy. The overall trend reflects the balance between the increasing nuclear attraction and the effect of electron shielding, which remains relatively constant in the same period. Thus, option C accurately describes the trend observed in ionisation energy across a period.