Ionization Energy

Ionization energy is the minimum energy required to remove an electron from an isolated atom, molecule, or ion in a specified state. Unless otherwise stated, the first ionization energy refers to a neutral species in its ground state. Electron removal produces a positively charged ion and a free electron. Ionization energy measures the energetic threshold for this process, rather than its probability or speed. The older term ionization potential remains in use, although the quantity is an energy rather than an electric potential. (goldbook.iupac.org)

Definition and units

For a neutral gaseous atom $X$, the first ionization process is

The threshold corresponds to the separated products with no excess kinetic energy, with the ion in its lowest accessible state. Specifying the initial state matters: an electronically excited atom requires less additional energy to reach a given ionization limit than its ground-state counterpart. Tabulated atomic values ordinarily describe ground-state species. (pml.nist.gov)

The SI unit of energy is the joule, but atomic and molecular ionization energies are commonly expressed in electronvolts (eV). Chemistry tables also use kilojoules per mole of particles. Multiplication by the Avogadro constant converts an energy per particle into a molar quantity; approximately, $1\ \mathrm{eV}$ per particle corresponds to $96.485\ \mathrm{kJ\,mol^{-1}}$. NIST’s atomic database supports several energy units, allowing the same threshold to be represented in different conventions. (goldbook.iupac.org)

Representative first ionization energies are approximately 13.5984 eV for hydrogen, 24.5874 eV for helium, and 5.1391 eV for sodium. These values illustrate substantial differences in how strongly different atoms bind their outermost electrons. (nist.gov)

Electronic structure and periodic trends

Ionization energies reflect electron configuration and the attraction between electrons and the atomic nucleus. Inner electrons partially shield outer electrons from nuclear charge. The spatial distribution of an atomic orbital, including its penetration into regions near the nucleus, also influences binding. Consequently, nuclear charge alone does not determine the ionization energy of a many-electron atom. (openstax.org)

Across a period of the periodic table, first ionization energy generally increases; down a group, it generally decreases as outer electrons occupy more extended shells. Alkali metals have relatively low first ionization energies, whereas noble gases have high values. These patterns are trends, not strict rules for every pair of neighboring elements. (openstax.org)

Two familiar exceptions reveal subshell structure. Boron has a lower first ionization energy than beryllium because its removed electron occupies a higher-energy 2p rather than 2s subshell. Oxygen’s value is lower than that of nitrogen: removing a paired 2p electron reduces electron–electron repulsion. Such differences make ionization energies useful evidence about electronic structure. (openstax.org)

Successive ionization energies

The second ionization energy concerns removal from the singly charged ion, not simultaneous removal of two electrons:

More generally, the $n$th value describes

Each threshold therefore belongs to a different initial charge state and electronic configuration. Atomic databases distinguish these stages explicitly. (goldbook.iupac.org)

Successive values increase, with especially large jumps when removal reaches core electrons. Calcium’s first three values are about 590, 1,145, and 4,912 kJ mol$^{-1}$; the third removes an electron from its closed-shell core. (openstax.org)

Molecular ionization

Molecules require a distinction between adiabatic and vertical ionization energies. The adiabatic value connects the neutral molecule’s ground state with the molecular ion’s ground vibrational state, allowing their equilibrium geometries to differ. The vertical value describes electron removal without changing nuclear geometry. Here, adiabatic is a spectroscopic designation, not simply the thermodynamic meaning of a process without heat exchange. (goldbook.iupac.org)

The distinction matters because ionization can alter molecular geometry and vibrational excitation. A measured spectrum can contain several bands associated with different electronic and vibrational states of the ion. A band’s onset may indicate an adiabatic threshold, but assigning that threshold requires attention to transition intensities and spectral structure. (webbook.nist.gov)

Measurement

Photoelectron spectroscopy determines ionization energies by using a photon of known energy and measuring the emitted electron’s kinetic energy. Neglecting recoil, energy conservation gives

where $h$ is the Planck constant, $\nu$ is photon frequency, $I$ is the ground-state ionization threshold, and $E_{\mathrm{exc}}$ is excitation energy retained by the ion. Thus, subtracting electron kinetic energy from photon energy identifies an ionization transition energy, not necessarily the lowest threshold. (webbook.nist.gov)

Other methods include analysis of converging spectral series and threshold-electron detection. Coincidence experiments detect an electron together with its corresponding ion, sometimes incorporating mass spectrometry to identify the product. NIST provides critically evaluated atomic thresholds and molecular ion-energetics data with information about experimental methods. (webbook.nist.gov)

Related quantities

Ionization energy differs from electron affinity, which concerns electron attachment or, equivalently, detachment from a negative ion. Both characterize gas-phase electron energetics, but they connect different charge states. (webbook.nist.gov)

It also differs from electronegativity, the tendency of a bonded atom to attract electrons. Ionization energy is a threshold for a specified isolated species; electronegativity describes electron attraction within a chemical bond and is expressed through comparative scales. (openstax.org)