Video narration
Why is pulling an electron from one atom harder than from another-like pulling a book from a tightly packed shelf? Ionisation enthalpy is the energy needed to remove one electron from an isolated gaseous atom. First ionisation enthalpy is for removing the first electron. It is not simply reactivity. It measures how tightly the nucleus holds the outer electron. So why the difference? Smaller atoms hold electrons closer, with stronger nuclear attraction, so removing one takes more energy.