1.4.1—Terms: first ionisation energy, IE

First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms, forming one mole of gaseous 1+ ions. It is measured under the stated standard conditions, in kJ mol⁻¹, and is positive because electron removal is endothermic.

Write the process for one atom as X(g) → X⁺(g) + e⁻, while the definition refers to one mole of gaseous atoms and one mole of electrons removed. The electron removed is the outer electron in the ground-state atom, so its attraction to the nucleus and the shielding by inner electrons affect the energy required.

Keep every state and amount in the definition: the atom and ion are gaseous, the product is a gaseous 1+ ion, and only one electron is removed in the first step. Do not write a solid ion, remove several electrons at once, or confuse first ionisation energy with a successive ionisation energy of an already positive gaseous ion.

Use this card for the definition and equation of first ionisation energy. Periodic trends, successive-ionisation jumps and detailed electronic-configuration deductions belong to the neighbouring objectives, even though they use the same gaseous-ion convention.