Negative ion

Negative ions are formed by the attachment of a supplementary electron to a neutral species. First, it may seem strange that this is possible, and the mechanism by which it can happen must be explained. Second, we may ask whether it is always possible, and how many electrons N(Z) can in fact be bound to a nucleus of charge Z. 

Taking the second point first, there are some rigorous results in this area. For example, it has been proved that, in nonrelativistic quantum theory, N(Z)/Z — 1 as Z — oo, a result which is only true for fermions.13 The Pauli principle plays a crucial role in its derivation [57]. Upper bounds on N(Z) have also been obtained. For an atomic nucleus, N(Z) Z. It follows that H2 is not stable. For a molecule, 

Negative ions were first studied by astrophysicists [59] who recognized that absorption by H is a major source of opacity in the infrared spectrum of the sun. Not all the elements may be able to form negative ions, although their nonexistence is hard to prove. Initially, it was believed (for plausible theoretical reasons) that atoms with closed outer subshells like the alkaline earths could not form negative ions. This has now been shown to be untrue, both experimentally [60] and theoretically [61]. 

The binding energy of negative ions is very small (usually of the order of 1 eV or less), which means that the potential is usually too weak to support an excited state. However, there are exceptions, such as Li-162] and Be- [63] and molecules such as NaGT, NaBr- and Nal- [64], and theory suggests there are several others [65], for reasons discussed in section 2.26. 

The fact that few negative ions with excited bound states exist does 

The negative ions of all Pt group elements M have been mass spectrometrically detected using ionization (by a pulsed rf vacuum-spark discharge) of powders of Ru, Rh, Os, and Ir and of metals (or alloys) of Pd and Pt [1]. 

Ground state configurations d s, expected [2] on the basis of regularities in the long periods, considered in more detail by Catalan et al. [3] and Edlen [4], were confirmed for Rh and lr , both k=8 [5], and Pt, k=9 [6]. For Ru and Os (probably k=7) no spectroscopic observations seem to exist, while for Pd the ground state configuration is 4d °5s [5]. For term designations, see below and the table of (atomic) electron affinities (p. 254). 

Photodetachment cross sections were also theoretically calculated for Rh and lr using a zero-core-contribution model The anion was described by a single electron loosely bound to a frozen core consisting of the neutral atom. Good agreement with the experimental spectra [5] was obtained [9]. A many-body calculation for Pd confirmed its existence with the 4d °5s configuration [10]. 

The Pt ion was produced in another sputter ion source, using positive ions from a discharge [6]. The source was earlier described in [11]. The photodetachment cross section (a) was measured in a crossed-beam experiment using a pulsed tunable dye laser. A threshold (at 17160 + 16 cm or 2.128+0.002 eV, see p. 254) was ascribed to the transition 

Fine-structure separations of ionic states were also estimated from extrapolation methods, thus for the F term of Rh and lr [5] and for the term of Pd [2, 5] and Pt [2, 6]. See also a more recent survey [12]. 

There are numerous negative ions that contain more than one atom. These are called complex or polyatomic ions. If the ion contains oxygen, another name is oxyanion. 

There are oxyanions that have very similar formulas but which differ in the number of oxygen atoms. The suffix ate is generally used when the maximum possible number of oxygen atoms are bonded to the non-oxygen atom. The suffix ite is used to name the ion containing fewer oxygen atoms. This usage is explained in our next rule  

Rule 8-When the oxyanions of an element can contain different numbers of oxygen atoms, the ion with fewer oxygen atoms gets the suffix ite, and the ion with the greater number of oxygen atoms gets the suffix ate. 

An oxyanion is a negative ion that contains one or more oxygen atoms. 

The following list shows the names of common ions that use the ite-ate nomenclature  

The dependence of et upon E has been found to be linear, at least at all bar the highest energies, for quite a number of polyatomic molecules [310, 369, 675]. It is clear from the slopes of these plots that eqn. (42) does not, in general, hold for dissociative resonance captures. 

It is perhaps also worth noting that the idea that tightness favours energy release to some extent runs counter to the other idea of lateness being a favourable factor. Lateness is generally a characteristic associated with loose transition states. 

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The Gouy-Chapman treatment of the double layer runs into difficulties at small Kx values when is large. For example, if is 300 mV, yo is 12 and if Co is, say, 10" mol/1, then the local concentration of negative ions near the surface, given by Eq. V-1, would be C = = 160 mol/1 The trouble... 

Nucleation in a cloud chamber is an important experimental tool to understand nucleation processes. Such nucleation by ions can arise in atmospheric physics theoretical analysis has been made [62, 63] and there are interesting differences in the nucleating ability of positive and negative ions [64]. In water vapor, it appears that the full heat of solvation of an ion is approached after only 5-10 water molecules have associated with... 

In the case of a sparingly soluble salt that dissociates into v positive ions M and negative ions A, the solubility S is given by... 

The above fomuilae for the absorption spectrum can be applied, with minor modifications, to other one-photon spectroscopies, for example, emission spectroscopy, photoionization spectroscopy and photodetachment spectroscopy (photoionization of a negative ion). For stimulated emission spectroscopy, the factor of fflj is simply replaced by cOg, the stimulated light frequency however, for spontaneous emission... 

Note that chemists tend to refer to positive ions as cations (attracted to the cathode m electrolysis) and negative ions as anions (attracted to an anode). In this section of the encyclopedia, the temis positive ion and negative ion will be used for the sake of clarity. 

Figure A3.5.3. The negative ion photoeleetron speetroineter used at the University of Colorado. The apparatus now eontains a UV-buildup eavity inside the vaeuiun system (not shown in this sketeh).

ZEKE (zero kinetic energy) photoelectron spectroscopy has also been applied to negative ions [M]. In ZEKE work, the laser wavelengdi is swept tlirough photodetachment thresholds and only electrons with near-zero kinetic energy are... 

On. CM, H O, and CO [31, 32, ]. A few negative ions have been studied using coaxial fast-ion/laser... 

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is tenued charge transfer or electron transfer. Proton transfer is also conunon in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detaclunent [73, 74],... 

These days, remarkably high-resolution spectra are obtained for positive and negative ions using coaxial-beam spectrometers and various microwave and IR absorption teclmiques as described earlier. Infonnation on molecular bond strengths, isomeric fonus and energetics may also be obtained from the teclmiques discussed earlier. The kinetics of cluster-ion fonuation, as studied in a selected-ion flow tube (SIFT) or by high-pressure... 

Photoelectron spectra of cluster ions yields cluster-bond strengdis, because each added ligand increases the bindmg energy of the extra electron in the negative ion by the amount of the ligand bond strength (provided the bond is electrostatic and does not appreciably affect the cln-omophore ion) [116]. 

Negative ions also have two unique thennodynainic quantities associated with them the electron affinity, EA, defined as the negative of the enthalpy change for addition of an electron to a molecule at 0 K [117. 121. 122]... 

The enthalpy for this process is the proton affinity of the negative ion. 

More complex ions are created lower in the atmosphere. Almost all ions below 70-80 km are cluster ions. Below this altitude range free electrons disappear and negative ions fonn. Tln-ee-body reactions become important. Even though the complexity of the ions increases, the detemiination of the final species follows a rather simple scheme. For positive ions, fomiation of H (H20) is rapid, occurring in times of the order of milliseconds or shorter in the stratosphere and troposphere. After fomiation of H (H20), the chemistry involves reaction with species that have a higher proton affinity than that of H2O. The resulting species can be... 

The negative ion chemistry is equally clear. N07(HNOO ,(HiO) ions are fomied rapidly. Only acids, HX,... 

The astrochemistty of ions may be divided into topics of interstellar clouds, stellar atmospheres, planetary atmospheres and comets. There are many areas of astrophysics (stars, planetary nebulae, novae, supemovae) where highly ionized species are important, but beyond the scope of ion chemistry . (Still, molecules, including H2O, are observed in solar spectra [155] and a surprise in the study of Supernova 1987A was the identification of molecular species, CO, SiO and possibly ITf[156. 157]. ) In the early universe, after expansion had cooled matter to the point that molecules could fonn, the small fraction of positive and negative ions that remained was crucial to the fomiation of molecules, for example [156]... 

Muschlitz E E 1957 Formation of negative ions in gases by secondary collision processes J. Appi. Rhys. 28 1414-18... 

Dessent C E FI and Johnson M A 1998 Fundamentals of negative ion photoelectron spectroscopy Fundamentals and Applications of Gas Phase Ion Chemistry ed K R Jennings (Berlin Kluwer)... 

Lineberger W C 1982 Negative ion photoelectron spectroscopy Applied Atomic Collision Physics, Vol 5, Special Topics ed FI S W Massey, E W McDaniel and B Bederson (New York Academic)... 

Mead R D, Stevens A E and Lineberger W C 1984 Photodetachment in negative ion beams Gas Phase Ion Chemistry ed M T Bowers (New York Academic)... 

Cordermann R R and Lineberger W C 1979 Negative ion spectroscopy Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and FI L Strauss (Palo Alto, CA Annual Reviews)... 

Esaulov A V 1986 Electron detachment from atomic negative ions Ann. Phys., Pahs 11 493-592... 

Miller T M, Leopold D G, Murray K K and Lineberger W C 1986 Electron affinities of the alkali halides and the structure of their negative ions J. Chem. Phys. 85 2368-75... 

Rosenbaum N H, Owrutsky J C, Tack L M and Saykaiiy R J 1986 Veiocity moduiation iaser spectroscopy of negative ions the infrared spectrum of hydroxide (OH ) J. Chem. Phys. 84 5308-13... 

MoFarland M, Albritton D L, Fehsenfeld F C, Ferguson E E and Sohmeltekopf A L 1973 Flow-drift teohnique for ion mobility and ion-moleoule reaotion rate oonstant measurements. III. Negative ion reaotions of 0 + CO, NO, FI2, and D, J. Chem. Phys. 59 6629-35... 

Bates D R 1991 Negative ions struoture and speotra Advances in Atomic, Molecular and Optical Physics ed D R Bates and B Bederson (New York Aoademio)... 

Bartmess J E 1998 Negative ion energetics data NIST Chemistry WebBook, NIST Standard Reference Database Number 69 ed W G Maiiard and P J Linstrom (Gaithersburg, MD Nationai institute of Standards and Technoiogy)... 

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