Ioniseringsenergien

Ioniseringsenergien, or ionization energy, is the amount of energy required to remove the most loosely bound electron from a neutral isolated atom in the gas phase, producing a positively charged ion. The first ionisation energy refers to removing the first electron; subsequent ionisations remove additional electrons from the resulting ion. In scientific contexts, ionisation energy is usually expressed per mole of atoms (kilojoules per mole) or per individual atom (electronvolts).

Ioniseringsenergien shows characteristic periodic trends. Across a period, it generally increases as nuclear charge grows with only gradual changes in shielding, making outer electrons harder to remove. Down a group, it decreases because added electron shells increase atomic size and shielding, reducing the effective attraction of the nucleus to the outer electrons. Irregularities occur due to electron configurations, such as half-filled or fully filled subshells. Noble gases exhibit particularly high first ionisation energies due to their stable electronic arrangements.

Second and higher ionisation energies are progressively larger, since removing an electron from a positively charged ion is more difficult than removing one from a neutral atom. This sharp rise reflects the stronger effective nuclear attraction after each removal and the increasing charge of the remaining ion.

Ioniseringsenergien can be measured by spectroscopic methods such as photoelectron spectroscopy. They are fundamental for understanding atomic structure, chemical reactivity, and periodic trends, and they underpin models of gas‑phase reactions and redox behavior.

Representative first ionisation energies (approximate): hydrogen about 1312 kJ/mol, helium about 2372 kJ/mol, sodium about 496 kJ/mol, neon about 2080 kJ/mol, chlorine about 1251 kJ/mol.