The energy needed to remove the outermost electron from a gaseous atom, generally increasing across a period.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
The first ionisation energy, IE1, is the energy needed to remove the outermost electron from a gaseous atom, as in M(g) to M+(g) plus e-. It generally increases across a period because Z_eff rises and the outer electrons are held more tightly. It falls down a group because the outer electron lies farther from the nucleus. Ionisation energy is related to metallic character, since metals have low IE values and readily form cations. It is measured in kJ/mol or electronvolts.
Key ideas: The energy to remove one mole of electrons from gaseous atoms: X(g) gives X+(g) + e-, in kJ per mol (H = 1312, He = 2372, Li = 520, Na = 496, Cs = 376). It generally increases across a period and decreases down a group. It is determined by Z_eff, distance, and the sublevel stability of the orbital.
Common mistakes and connections: Students forget that ionization is endothermic (energy is absorbed), or that it must involve gaseous atoms. Another error is claiming that IE rises smoothly across a period; there are dips at B and O. It controls metallic character and cation formation, and links to the Born-Haber cycle.
Ionisation energy peaks at each noble gas and drops sharply for the next alkali metal, giving the periodic saw-tooth pattern. The interactive lesson shows this as a live animation or chart.
The first ionisation energy of sodium is 496 kJ/mol, while that of neon is 2081 kJ/mol.
The energy needed to remove the outermost electron from a gaseous atom, generally increasing across a period.
Ionization Energy is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Atomic Structure & Periodicity first: Atomic Radius, Ionic Radius, Isoelectronic Species.
The next topics in Atomic Structure & Periodicity are Successive Ionization Energies and Anomalies in Ionization Energy.
Yes. The interactive lesson includes a self-check question on Ionization Energy, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Ionization Energy is topic 86 of 115 in Atomic Structure & Periodicity.