Ionisation

Ionisation changes a neutral atom or molecule into an ion by altering its number of electrons. Removing an electron produces a positively charged cation, while adding an electron produces a negatively charged anion.¹ The nucleus remains essentially unchanged in ordinary chemical ionisation: protons are not created or destroyed. The required energy is called the ionisation energy, and it depends on the species, its electronic configuration, and the particular electron removed.

For an isolated hydrogen atom in its ground state, removing the electron requires 13.6 electronvolts. Successive ionisation energies generally rise because each remaining electron is held more strongly by the increasingly positive ion. Electron gain can instead release energy or require energy, depending on the atom and the stability of the resulting configuration.

Ionisation can be caused by radiation, collisions, heat, electric fields, or chemical reactions. Ultraviolet and X-ray photons can eject electrons when their energy exceeds an ionisation threshold; this is photoionisation.² Fast electrons, ions, or particles can transfer enough energy during collisions to remove electrons, producing cascades in gases and solids.

In a flame or a hot plasma, thermal collisions generate a population of ions and free electrons. Strong electric fields can also cause field ionisation, particularly near sharp surfaces. In chemistry, electron transfer between atoms or molecules is often described as ionisation when it creates charged species, although proton transfer and ionic dissociation are distinct processes rather than simple electron removal.

Ionisation is central to plasmas, atmospheric electricity, radiation detection, and analytical chemistry. A plasma contains substantial numbers of free charged particles whose collective behaviour differs from that of an ordinary gas.³ In the upper atmosphere, solar ultraviolet radiation and energetic particles ionise gases, contributing to the ionosphere and to auroral phenomena.

Ionisation chambers measure radiation through the electrical charge created in a gas, while Geiger–Müller tubes use a much larger avalanche of secondary ionisation. In mass spectrometry, ionisation converts sample molecules into gas-phase ions so that their mass-to-charge ratios can be measured; electron ionisation, chemical ionisation, electrospray, and matrix-assisted laser desorption are distinct methods.⁴

Ionisation is not always a complete or permanent transformation. An excited atom may lose an electron through autoionisation, in which internal electronic energy is redistributed to eject another electron. An ion can also recombine when it captures an electron, or when positive and negative ions neutralise one another.

Ionisation energies are not perfectly smooth across the periodic table: subshell arrangements and electron pairing create notable exceptions. Molecular ionisation can fragment the parent molecule, so the ion detected in a mass spectrometer may be a molecular ion, a protonated molecule, or a structurally informative fragment. In condensed matter, an ionisation event may create an electron–hole pair rather than a freely moving ion, a distinction important in semiconductors and radiation sensors.⁵

Ion

An atom or molecule with a net electric charge because its electron number differs from its proton number.

Cation

A positively charged ion formed, in ordinary ionisation, by electron loss.

Anion

A negatively charged ion formed by electron gain.

Ionisation energy

The minimum energy required to remove a specified electron from an isolated gaseous atom or molecule.

Recombination

A process in which free electrons and positive ions combine, reducing the degree of ionisation.