Ion and Ionization

Ionization energy

Ionization methods

Resources

Ionization is the removal from, or addition to, one or more electrons from an atom or molecule, thereby creating a particle with a nonzero electrical charge—an ion. The word “ionization” is also used for the process in which an ionic solid, such as a salt, dissociates into its component ions in solution. This article will discuss primarily positive ionization, which occurs when an atom or molecule loses electrons. This is the most common form of ionization.

In order to remove an electron from an atom, enough energy must be supplied to break the bond between the negatively charged electron and the positively charged nucleus; this is the ionization energy. Ionization can be induced by high energy radiation such as x rays and ultraviolet light (photoionization), bombardment by high energy electrons (electron impact ionization) or small molecular ions (chemical ionization) and by exposure to high electric fields (field ionization). Ionization is employed in many important analytical techniques used to study the character of atoms and molecules including mass spectrometry, photoelectron and Auger electron spectroscopy, and multiphoton ionization spectroscopy.

Ionization energy

In the Bohr model of atomic structure, electrons orbit the nucleus at fixed distances, similar to the orbits of the planets around the sun. For every element, the distances of the electron orbitals are fixed and unique to that element. Normally, the electrons occupy the orbits closest to the nucleus. This is the most stable configuration of the atom and is known as the ground state. To move an electron to an orbital further from the nucleus requires the input of energy. Atoms which have an electron in a higher orbit are said to be in an excited state.

The strength of attraction between a negatively charged electron and the positively charged nucleus is greater the closer together they are. The energy needed to move an electron from one orbit to a higher energy one is equal to the difference in the attraction between the two configurations; it takes increasing amounts of energy to move an electron to orbits further and further from the nucleus. The energy needed to move electrons from one orbit to another can be thought of like the energy needed to move between rungs on a ladder; to move from a lower rung to a higher rung requires the input of energy, and the more rungs you move up, the more energy it takes. However, if the electron is moved too far from the nucleus, the attraction between the electron and the nucleus is too small to hold the electron in its orbit, and, analogous to stepping of the top rung of the ladder, the electron is separated from the atom leaving behind a positively charged atom; the atom has been ionized.

The ionization of an atom can be represented by:

X + energy → X⁺ + e^-

where X is a single atom of any element and e<+>- is the ejected electron. The amount of energy required for this process is called the ionization energy. The ionization energy is a measure of how difficult it

is to remove the electron from the atom—the more strongly the electron is attracted to the nucleus, the higher the ionization energy. Although in theory it is possible to remove any of the electrons from an atom, in practice, the electron in the outermost orbit is typically the first to be removed. The energy required to remove the first electron is called the first ionization energy.

For many electron atoms it is possible to remove more than one electron. A second electron can be removed from a singly charged ion x⁺ to yield a doubly charged ion, x²⁺. This process can be written as:

X⁺ + energy → x²⁺ + e^-

The energy required to remove the second electron is called the second ionization energy. Following the removal of the first electron, the atom has one more positively charged proton in the nucleus than it has negatively charged orbiting electrons. This charge imbalance causes the remaining electrons to be held even more tightly to the nucleus. Consequently, more energy is required to remove the second electron than was required to remove the first. The removal of subsequent electrons, creating x³⁺, X⁴⁺, and so on, requires ever increasing amounts of energy. This effect is rather like a small child with a collection of toys. The child might be easily persuaded to give the first toy away, but will hold on to each remaining toy with increasing vigor, thereby requiring increasing amounts of persuasion to give away each subsequent toy.

The first and second ionization energies of the elements in the first three rows of the periodic table are listed in Table 1. Note that the second ionization energy in all cases is larger than the first ionization energy. The hydrogen atom, however, having only one electron, only has a first ionization energy. Note that the ionization energy, in general, increases with increasing atomic number for elements within the same row of the periodic table. The ionization energy is smallest for the alkaline earth elements, Li, Na, K, etc., increasing with atomic number and reaching a maximum at the end of each row, corresponding to the noble gases, Ne, Ar, Kr, etc. This effect is related to the way in which atomic orbitals are filled. The noble gases have filled electronic orbitals, which are very stable.

Molecules can be ionized in a manner analogous to atoms. However, because electrons form the bonds that hold molecules together, their removal may result in the bond being weakened, or even broken. The ionization energies of some simple molecules are listed in Table 2. Note that in general, the ionization energies of molecules have values the same order of magnitude as the first ionization energies of isolated atoms. molecules

with only a few atoms, such as N₂, cO₂ and H₂O, tend to have the highest ionization energies. Within a group of similar molecules, such as the alkanes listed in the table, the ionization energy decreases with increasing size. This effect is due to the fact that in larger molecules, there are more electrons available for ionization without disrupting the bonding stability of the molecule. Again, this is analogous to persuading a child to give up its toys; the more toys the child has, the easier it will be to persuade it to give one up.

Ionization methods

Ions, being electrically charged, are much easier to manipulate and detect than electrically neutral atoms or molecules. The direction or speed of ions can be changed by application of electric and magnetic fields, similar to the way a magnet can move small pieces of a magnetic material. Ions can be detected simply by measuring the electric current produced by their movement. Consequently, ionization is frequently employed in scientific apparatus to transform neutral species to charged species so that they may be more easily studied.

Mass spectrometry is a powerful analytical technique based on the transformation of the neutral components of a sample to ions which are then separated according to their mass-to-charge ratio. The structure and composition of molecular species can be deduced by studying the masses of the molecular ion along with the smaller, fragment ions, which are sometimes formed. There are several different types of ion sources that are used in mass spectrometers: electron impact, field ionization and chemical ionization are the most common, and are described below. Other instrumental analytical techniques which are based on ionization include spectroscopies which study the energy of the ejected electron as well as the positively charged ion. These methods typically employ electromagnetic radiation to supply the ionization energy (photoionization). The basic requirement for all ionization sources is that sufficient energy must be supplied to remove at least one electron.

Electron impact source

The most common method of producing ions for mass spectrometry is by bombarding a gaseous sample with a stream of fast moving electrons. The stream of electrons, produced by an electron gun (a heated tungsten wire from which electrons are emitted—thermionic emission), bombard the sample and “kick” out additional electrons. The process of electron impact ionization is not very efficient. Because of the very small size of electrons and the relatively low density of electrons around molecules, electron-electron impacts are rare. Nonetheless, electron impact is the most widely used ion source in commercial mass spectrometers. Electron guns can produce vast quantities of electrons, so even if one in a million is successful, enough ions can be generated. When the impact is effective in producing ionization, typically there is more energy supplied by the impact than is needed to remove the electron. The excess energy may result in the ion breaking up into smaller fragment ions. The intact molecular ion is referred to as the parent ion, and the fragment ions are called daughter ions.

Field ionization

Ionization may also be produced by subjecting a molecule to a very intense electric field. This process is called field ionization. A familiar example of field ionization is the small blue spark that jumps from the tip of your finger to any grounded surface on a dry day when static electricity can build up. The strong electrostatic field actually pulls electrons out of your finger. Electric fields are strongest at the tips of pointed conductors. To produce electric fields of sufficient magnitude to ionize molecules, very fine, sharpened wires are used. Field ionization sources are relatively gentle compared to electron impact sources in that they do not deposit as much excess energy into the parent ion. Therefore, field ionization sources are usually employed when we do not want to damage the ionized specimen too much.

Chemical ionization

Chemical ionization is similar to electron impact ionization except that a beam of positively charged molecular ions, rather than electrons, is used to bombard and ionize the sample. The bombarding ions are usually small molecules such as methane, propane, or ammonia. Because of the much larger size of a molecular ion compared to an electron, these collisions are highly reactive and generally produce less fragmentation than electron impact ionization with comparable efficiency. Chemical ionization is widely used in commercial mass spectrometers, and many instruments are equipped with a source which is capable of both electron impact and chemical ionization.

Photoionization

If the ionization energy is supplied by electromagnetic radiation, the ionization is called photoionization, referring to the fact that a photon of radiation produces the ionization. However, not all electromagnetic radiation has sufficient energy to cause ionization. Generally, only radiation with wavelengths shorter than visible light, that is, radiation in the ultraviolet, x ray, and gamma ray regions of the electromagnetic spectrum can produce ionization.

Ultraviolet radiation can cause ionization of many small molecules, including oxygen, O₂. In fact, short wavelength solar radiation causes ionization of molecular oxygen and molecular nitrogen found in the upper atmosphere; these processes are important to the chemistry of Earth’s atmosphere. In the laboratory, ultraviolet light from special lamps or lasers is used to ionize molecules in order to study them. Ultraviolet photoelectron spectroscopy (UPS) measures the energy of the departing electron.

The high energy carried by x rays can easily cause ionization of isolated atoms. X rays are therefore frequently referred to as ionizing radiation. X ray photoelectron spectroscopy and Auger spectroscopy are two techniques which, like ultraviolet photoelectron spectroscopy, study the ejected electron to gain information about the atom from which it came.

KEY TERMS

Chemical ionization —A method of producing ions by bombarding a sample with a stream of positively charged ions.

Electron impact ionization —A method of producing ions by bombarding a sample with a stream of electrons.

Field ionization —A method of producing ions by exposing the sample to a very intense electric field.

Flame ionization —A method of producing ions by exposing the sample to a flame of a very high temperature.

Ion —An atom or molecule which has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).

Ionization energy —The amount of energy required to completely remove an electron from an atom or molecule, thereby creating a positively charged ion.

Photoionization —The production of ions by exposing a sample to electromagnetic radiation, usually ultraviolet or x-ray radiation.

Flame ionization

Probably the simplest way of supplying energy for ionization is by subjecting the atoms or molecule to a flame. However, temperatures of several thousand degrees are usually required to achieve an appreciable degree of ionization. Specialized flames, such as an electrical arc, spark or plasma, can produce the necessary temperatures in a controlled manner.

See also Dissociation.

Resources

BOOKS

Brown, Ian G. The Physics and Technology of Ion Sources. New York: Wiley-VCH, 2004.

Maier, Joachim. Physical Chemistry of Ionic Materials: Ions and Electrons in Solids. New York: John Wiley & Sons, 2004.

Oxtoby, David W., et al. The Principles of Modern Chemistry. 5th ed. Pacific Grove, CA: Brooks/Cole, 2002.

PERIODICALS

Rainville, Simon, et al. “An Ion Balance for Ultra-High-Precision Atomic Mass measurements.” Science. 303 (2004): 334-338.

Karen Trentelman