ion, ion- +
(Greek: ion, "going"; neuter present participle of ienai, "to go"; because an ion moves toward the electrode of an opposite charge)
Positively charged ions are called cations; negatively charged ions are labeled, anions.
Ions are formed by the addition of electrons to, or the removal of electrons from, neutral atoms or molecules or other ions; by combination of ions with other particles; or by rupture of a covalent bond between two atoms in such a way that both of the electrons of the bond are left in association with one of the formerly bonded atoms.
Examples of these processes include the reaction of a sodium atom with a chlorine atom to form a sodium cation and a chloride anion; the addition of a hydrogen cation to an ammonia molecule to form an ammonium cation; and the dissociation of a water molecule to form a hydrogen cation and a hydroxide anion.
Many crystalline substances are composed of ions held in regular geometric patterns by the attraction of the oppositely charged particles for each other.
Ions migrate under the influence of an electrical field and are the conductors of electric current in electrolytic cells.
This can be a highly efficient method and has the experimental advantage of producing ions with a small energy spread characteristic of the filament temperature, typically a few tenths of an electron volt, as compared with beam energies of thousands of electron volts.
The filaments, generally made of platinum, rhenium, tungsten, or tantalum, are heated by current.
Surface ionization requires a nearby source of atoms, often another filament operating at lower temperatures.
Samples can also be loaded directly on the filament, a widely used and successful technique and one that has resulted in many interesting chemical treatments of the sample when it is deposited on the filament.
One such application changed lead from a difficult to an easy element to analyze, enabling important geochronological and environmental measurements.
A disadvantage of thermal ionization is the possible change in isotopic composition during the measurement. This effect is caused by Rayleigh distillation, wherein light isotopes evaporate faster than heavy ones.
Studies done on isotopes that come from radioactive decay; such as, those used in determining the ages of rocks, encounter this problem, but it is correctable using the measured values of the isotopes that are not radiogenic.
With few exceptions the use of a thermal source requires the chemical separation of the sample. Useful data are commonly obtained on extremely small (nanogram) samples.