Ionization is the formation of ions by the removal or addition
of one or more electrons from the outer
parts of atoms. It occurs by ionizing
radiation or if an atom suffers a sufficiently violent collision.
The ionization potential is the minimum amount of energy needed to remove an electron to infinity from the ground state. If the electron has already been excited to a higher level, less energy is needed to remove it. Where an atom has two or more electrons, the ionization potential for the second and subsequent electrons is greater than for the first electron.
The stripping away of electrons can happen when atoms are exposed to short wavelength electromagnetic radiation as in the outer atmosphere of stars hotter than the Sun or in bright nebulas. Matter that has been completely ionized so that it is a mixture of bare nuclei and free electrons, as in the intensely hot interior of stars, is known as plasma. Some substances are made up of natural ions (for example, table salt – sodium chloride – contains Na+ and Cl- ions), held together by electrical attraction, which separate when dissolved in water, enabling the solution to conduct electricity.
Astronomers identify an atom that has lost a single electron by the Roman numeral II (the neutral atom, by I). In other areas of science, the superscript +. For example, neutral hydrogen is denoted HI and ionized hydrogen by HII or H+. Doubly ionized He (helium that has lost both electrons) is denoted by He III or He2+. Very high degrees of ionization occur in some astrophysical settings (e.g., very- high-temperature gases).
Ions that retain at least one bound electron can absorb or emit radiation, so producing spectral lines (emission and absorption lines) that differ in wavelength from those produced by neutral atoms. Photons are also emitted when an ion captures, or recaptures, an electron – a process called recombination.