Ionization energy

Ionization energy is the minimum energy required to remove an electron from an isolated gaseous atom, molecule, or ion. Successive ionization energies measure the energy needed to remove additional electrons.

Photoionization: The removal of an electron from an atom or molecule by absorption of a photon. Photons supply the energy needed to exceed an ionization threshold.
Electronvolt: A unit of energy equal to the energy gained by an elementary charge across one volt. Ionization energies are commonly reported in electronvolts per particle.
Mass spectrometry: A technique that identifies ions by measuring their mass-to-charge ratios. Ionization methods create the charged particles that mass spectrometers analyze.
Electron affinity: The energy change when a neutral gaseous atom gains an electron. It concerns electron gain, whereas ionization energy concerns electron removal.
Hydrogen: The chemical element with atomic number 1, whose neutral atom contains one proton and one electron. Its single-electron atom provides the simplest ionization-energy case.
Coulomb's law: The law describing the electrostatic force between charged particles. Electrostatic attraction between the nucleus and electron helps set the removal energy.
Atomic number: The number of protons in an atom's nucleus. It sets nuclear charge and helps explain periodic changes in electron binding.
Photoelectron spectroscopy: A technique that infers electron binding energies by measuring electrons emitted after photon absorption. It measures ionization energies and reveals electronic structure.
Work function: The minimum energy needed to remove an electron from a solid surface. It describes electron escape from a material, not an isolated gaseous species.
Helium: The chemical element with atomic number 2 and a filled first electron shell. Its compact, filled shell gives it an exceptionally high first ionization energy.

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