The amount of energy required to remove an electron from a mole of atoms in the gas phase is called:

Ionization energy (also called ionization enthalpy) is the minimum energy needed to remove the most loosely held electron from an isolated gaseous atom in its ground state, forming a positive ion. When it is measured for one mole of atoms, it is expressed in kJ per mol. The atoms must be in the gaseous state so that the attraction of neighbouring atoms does not affect the value.

The energy needed to remove the first electron is the first ionization energy. Removing a second electron from the positive ion needs more energy, so the second ionization energy is always higher than the first, because the electron is now being pulled away from a positively charged ion.

Ionization energy follows a clear pattern in the periodic table:

• Across a period (left to right), it generally increases, because the nuclear charge increases and atoms become smaller, holding electrons more tightly.
• Down a group, it decreases, because atoms become larger and the outer electrons are farther from the nucleus and shielded by inner shells.

Noble gases have the highest ionization energies in their periods, because of their stable filled shells. Helium has the highest first ionization energy of all elements. Alkali metals, such as sodium and caesium, have the lowest values, which is why they lose electrons easily and are very reactive.

Why not the other options?

• ✗Electron affinity is the energy released when an electron is added to a neutral gaseous atom; chlorine has the highest electron affinity.
• ✗Electronegativity is the tendency of an atom in a molecule to attract shared electrons; fluorine is the most electronegative element.
• ✗Lattice energy is the energy released when gaseous ions combine to form one mole of an ionic solid.