Electrons affinity

The electron afimity, Eea. is the difference in energy between a neutral atom and its anion. It is the energy released in the process 

The electron affinity is positive if the anion has a lower energy than the neutral atom. 

We use the convention that Eea 0 signifies a positive affinity for the added electron. Distinguish the electron affinity from the electron-gain enthalpy, which is negative for such an exothermic process (that is, has the opposite sign to the electron affinity, and differs very slightly in value). 

Here we explore how the ionic radius and charge CcUi work together to impcirt unique chemical properties to an ion, leading to unique biocheruiccil fimction. Consider the Zn + ion, which is foimd in the active sites of mcuiy enzymes. An example is carbonic anhydrase (Atlcis P2), which catalyzes the hydration of CO2 in red blood cells to give bicarbonate (hydrogenccirbonate) ion  

To understand the catalytic role played by the Zn ion, we need to know that a Lewis acid is an electron-poor species that forms a complex with a Lewis base, an electron-rich species. Metal cations are good Lewis acids, and molecules with lone pairs of electrons, such as H2O, are good Lewis bases. 

TABLE 5.8 Electron affinities (ici/moi) for seiected eiements. For O and S, both the first and second E.A.s are iisted. 

Vertical exceptions within a group also occur. The E.A. for F is less than that for Cl, for example. The smaller-than-expected E.A. for fluorine can be rationalized because of fluorine s extremely small radius. The addition of an electron to its valence shell would therefore increase the magnitude of the electron-electron repulsions. Consequently, the E.A. for fluorine is somewhat less than that for chlorine. Additionally, the bond dissociation enthalpy for Fj (155 kj/mol) is considerably less than that expected based on the other members of its group. For comparison, Clj, Br2, and I2 have bond dissociation enthalpies of 242, 193, and 151 kj/mol. Other anomalies occur for N and O, whose electron affinities are also less than the group trend would have predicted. By analogy with the F-F bond strength, the N-N and 0-0 bonds are likewise weaker than those for P-P or S-S. In fact, both the hydrazine (N-N) and peroxide (0-0) classes of compounds are particularly reactive. Hydrazine, N2H4, was once used as a rocket fuel, and many peroxides are potentially explosive. 

Electron affinity (EA) is the energy released (the negative of the enthalpy change A/7) when an atom in the gas phase accepts an electron. Consider the process in which a gaseous chlorine atom accepts an electron  

Just as more than one electron can be removed from an atom, more than one electron can also be added to an atom. While many first electron affinities are positive, subsequent electron affinities are always negative. Considerable energy is required to overcome the repulsive forces between the electron and the negatively charged ion. The addition of two electrons to a gaseous oxygen atom can be represented as  

The term second electron affinity may seem like something of a misnomer, because an anion in the gas phase has no real affinity for an electron. As discussed in Chapter 8, a significantly endothermic process such as the addition of an electron to a gaseous O ion happens only in concert with one or more exothermic processes that more than compensate for the required energy input. 

Sample Problem 7.5 lets you practice using the periodic table to compare the electron affinities of elements. 

For each pair of elements, indicate which one you would expect to have the greater first electron affinity, 4, (a) A1 or Si, (b) Si or P. 

Jnst as more than one electron can be removed from an atom, more than one electron can also be added to an atom. While many first electron affinities are positive, subsequent electron 

Student Annotation Although IE and EA both increase from left to right across a period, an increase in IE means that it is less likely that an electron will be removed from an atom. 

An increase in 4, on the other hand, means that it is more likely that an electron will be accepted by an atom. 

F ig U re 7.10 (a) Electron affinities (kJ/mol) of the main group elements, (b) Electron affinity as a 

Why are the electron affinities of the Group 4A elements more negative than those of the Group 5A elements  

The first ionization energy of an atom is a measure of the energy change associated with removing an electron from the atom to form a cation. For example, the first ionization energy of Cl( ), 1251 kj/mol, is the energy change associated with the process 

The positive value of the ionization energy means that energy must be put into the atom to remove the electron. All ionization energies for atoms are positive Energy must be absorbed to remove an electron. 

It is important to understand the difference between ionization energy and electron affinity Ionization energy measures the energy change when an atom loses an electron, whereas electron affinity measures the energy change when an atom gains an electron. 

The greater the attraction between an atom and an added electron, the more negative the atom s electron affinity. For some elements, such as the noble gases, the electron affinity has a positive value, meaning that the anion is higher in energy than are the separated atom and electron  

The electron affinity increases or in other words, the negativeness of the electron affinity increases from left to right along a period. 

The electron affinity of an atom can be defined as the energy change which takes place when an electron is added to an atom (+ve when energy is given out). It is the reverse of ionisation (Table 3.4). 

General trends in electron affinities of A group elements with position in the periodic table. There are many exceptions. 

The convention is to assign a negative value when energy is released and a positive value when energy is absorbed. Most elements have no affinity for an additional electron and thus have an electron aflinity equal to zero. We can represent the electron affinities of helium and chlorine as 

Electron affinity involves the addition of an electron to a neutral gaseous atom. The process by which a neutral atom X gains an electron 

Unless otheiwise noted, all content on this page is Cengage Learning, 

Elements with very negative electron affinities gain electrons easily to form negative ions (anions). 

FIGURE 2-13 Ionization Energies and Electron Affinities. Ionization energy = At/ for 

Electron affinity can be defined as the energy required to remove an electron from a negative ion  

FIGURE 2.13 Ionization Energies and Electron Affinities. Ionization eneigy = AU for M(g)------ M+(g) + e 

Hotop and W. C. Lineberger, J. Phys. Chem. Ref Data, 1985,14,731). Numerical values are in Appendices B-2andB-3. 

Similar patterns appear in the other periods, for example Na through Ar and K through Kr, omitting the transition metals. The transition metals have less dramatic differences in ionization energies, with the effects of shielding and increasing nuclear charge more nearly in balance. 

FIGURE 2.14 First and Second ionization Energies and Electron Affinities 

Many atoms have a tendency to add one or more electrons when forming compounds. In most cases, this is an energetically favorable process. As will be described in Chapter 4, one step in forming an ionic bond is the addition of an electron to a neutral, gaseous atom to give a negative ion, which can be shown as 

The addition of an electron to an uncharged atom or negatively charged ion is referred to as the electron addition enthalpy. The energy associated with removal of an electron from a negatively charged species (the atom that has gained an electron) is the electron affinity. 

In most cases, the enthalpy associated with this process is positive, meaning that energy is required to remove the electron from the atom that has gained it. Most atoms add one electron with the release of energy, but when O2- and S2 are formed, the atom must add two electrons. The addition of a second electron is always unfavorable. There is no atom that will add two electrons with a release of energy. Therefore, in forming compounds that 

Experimentally, the electron affinity is difficult to measure, and most of the tabulated values are obtained from thermochemical cycles where the other quantities are known (see Chapter 4). Electron affinities are often given in units other than those needed for a particular use. Therefore, it is useful to know that 1 eV molecule-1 = 23.06 kcal mole-1 and 1 kcal = 4.184 kJ. Electron affinities for many nonmetallic atoms are shown in Table 2.3. 

Note that the highest values correspond to the Group VIIA elements. 

In mass spectrometry, electron affinity (EA) is a useful factor for working in electron attachment, as we will see in Section 9.5.1. By definition, electron affinity is the energy that must be supplied to a negative ion to remove an electron from it. It corresponds to the AH of the reaction X — X -1- e. Note that in this case AH = AG because the variation of entropy AS of the system is nil. To estimate the feasibility of the capture of an electron by a molecule, one must consider the electron affinity of the corresponding negative ion the higher the affinity is, the easier the capture of an 

Source Harrison, A. G. 1992. Chemical Ionization Mass Spectrometry. Boca Raton, FL  

Ionization energy is defined as the energy required to remove an electron from an atom, forming a positively charged cation. But we also know that some atoms will routinely pick up an electron to form a negatively charged anion. 

The first four ionization energies (all in kj/mol) for the elements of the first three periods. Those ionizations with values shown in shaded cells with bold print involve removing the last electron from a particular shell. Further ionization requires removing an electron from a more stable filled shell, and this leads to a very large increase in ionization energy. 

The study ofener changes in chemical reactions is called thermodynamics and is presented in Chapters 9 and 10. 

Closer examination of the electron affinity values reveals that there are exceptions in the trends associated with electron pairing, just as there were for ionization energy. Thus, nitrogen does not have a negative electron affinity because adding an electron would force the pairing of electrons in a orbital, a process that increases the electron-electron repulsion substantially. 

Although Thomas Edison is often regarded as the inventor of the electric light bulb, the first patent for an incandescent lamp was granted to Frederick de Molyens of London in 7 841. 

Two experimental results are available for the adiabatic electron affinity which were obtained by photodetachment studies on PH ions. A=1.028 0.010 eV was obtained from a laser (488 nm) photoelectron spectrum of PH after rotational corrections had been made to the detachment energy measured from the center of the PH(X v=0) PH (X rij, v = 0) peak [1]. A less precise result, A=1.00 0.06 eV, has been obtained from the lowest threshold energy of the photodetachment cross section measured between 0.8 and 2.8 eV (1.5 to 0.4 xm a second threshold at -1.9 eV obviously corresponds to the PH(a A) PH (X rij) transition) [2]. 

The same authors reported in a recent work the PES spectra of five MgC met-cars (M = Ti, V, Cr, Zr, In the 3d transition series, the observed electron 

3 Collective electronic properties delayed ionization and delayed atomic ion emission 

Recommended values from a recent compilation [1] are shown below, together with the atomic (M) and ionic (M ) ground states involved and the determination methods used (LPES = laser photodetachment electron spectroscopy, LPT = (tunable) laser photodetachment threshold, SE=semiempirical extrapolation). For original work and details, see the remarks. 

A previous compilation [6] contained semiempirical estimates for Ru, Rh, and Pd (1.1, 1.2, and 0.6, all +0.3 eV), which were based on two kinds of extrapolation methods, i.e., 1) the isoelectronic extrapolation (number of electrons N fixed), used in [7], and 2) the horizontal analysis (degree of ionization q fixed Z = N+q), used in [3] and improved in [8]. Semiempirical data for Os and Ir (1.1+0.3 and 1.6+0.2eV) [6] were based only on the second method. Estimates for Rh, Pd, and Pt were earlier obtained from linear plots of the square roots of ionization potentials vs. q (for each element) [9]. 

Theoretical calculations were performed by the spin-restricted relativistic Hartree-Fock-Slater transition-state method, yielding for the series Ru through Pt (transition states in parentheses) A = 1.81 (5s ), 1.80 (5s ), 1.07 (5s g), 1.75 (5d ), 2.38 (5d g), and 2.77 (6s ) 

Check Compare your result with the data shown in Table 8.2. In (a), is your prediction consistent with the fact that the metallic character of the elements increases as we move down a periodic group In (b), does your prediction account for the fact that alkali metals form +1 ions while alkaline earth metals form +2 ions  

Practice Exercise (a) Which of the following atoms should have a larger first ionization energy N or P (b) Which of the following atoms should have a smaller second ionization energy Na or Mg  

Label the plots shown here for the first, second, and third ionization energies for Mg, Al, and K. 

Another property that greatly influences the chemical behavior of atoms is their ability to accept one or more electrons. This property is called electron affinity, which is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion. 

Consider the process in which a gaseous fluorine atom accepts an electron  

The first ionization potentials of fullerene molecules C decrease from 7.57 to 6.92 eV as n increases from 60 to 106 [35]. Fullerenes, especially those with 60, show a behavior similar to that of a simple charged sphere. An extrapolation of I of these fullerenes to n - oo gives = 5.13 eV, i.e. the work function of the graphite monolayer (graphene). This effect is manifest also in the fullerides M (C6o) where M = Sc, Ti, V, Cr and x= 1, 2, 3 the ionization potentials are generally lower than in free fullerenes, and further decrease from 6.4 eV for = 1 to 5.7 eV for n = 3. A similar decrease of the ionization potentials has been observed on complexation of the same metals with benzene, I decreasing as n and m in the Mn(C6H6)m complex increase (Table 1.2) [33], 

Ionization potentials of Si clusters show a similar, but weaker, dependence on n [36]  

Another atomic characteristic very important for understanding the nature of the chemical bond is the electron affinity (A), i.e. the amount of energy released when one electron is added to a neutral atom. Thus, A is equivalent to the ionization potential of a negatively charged atom, or a zero ionization potential . 

Electron affinities are intrinsically much more difficult to measure than ionization potentials. In fact, all determinations before 1970 were indirect and unreliable. Today, the principal experimental technique uses the photoelectric effect. A beam of anions is crossed with a light (laser) beam and the frequency is recorded at which the anion dissociates and scattered electrons occur [38]. The most accurate experimental values of A are listed in Table 1.3. At present, there is no practical way of measuring negative electron affinities, which are only available from theoretical calculations. Thus, a universal method to calculate A2 of atoms and molecules has been suggested by von Szentpdly [39], 

Note that hydrogen is similar to alkali metals in its electron affinity, and to halogens in its ionization potential (see Table 1.1), in accordance with the traditionally ambiguous placing of H in the Periodic Table in either Group 1 or Group 17. 

Theoretical values obtained by various versions of the CNDO method [4 to 6] and the Pariser-Parr-Pople method [7] vary between -0.89 and 6.28. 

4 Dipole Moment i in D. Quadrupole Moment O. Octupole Moment Q 

The only experimental value, = ( )0.136 0.01 (4.5xl0 = C m), was obtained from the Stark effect on two components of the 1i,o 1o,i multiplet in the microwave spectrum of NFg 

A few ab initio SCF-MO calculations showed that i is very sensitive to changes in basis set. Minimal basis sets of Slater and of Gaussian lobe functions gave i = +0.656 [2, 3] and +0.506 

Semiempirical CNDO [7 to 9], INDO [8,10], and MNDO [11] calculations predicted moments between 0.0035 and 0.85 without elucidating the direction. However, the direction of the dipole moment was included in the results =-0.12 (CNDO) and -0.38 (INDO) [12,13]. 

Autodetachment spectra of and ion beams upon IR laser excitation of 

TABLE 8.1 Successive Values of Ionization Energies for the Elements Sodium through Argon (kJ/mol) 

As shown in Table 8.1, similar trends exist for the successive ionization energies of many elements. The ionization energy increases fairly uniformly with each successive removal of an outermost electron, but then takes a large jump with the ranoval of the first core electron. 

Based on what you just learned about ionization energies, explain why valence electrons are more important than core electrons in determining the reactivity and bonding in atoms. 

Electron affinity and metallic character also exhibit periodic trends. Electron affinity is a measure of how easily an atom will accept an additional electron and is crucial to chemical bonding because bonding involves the transfer or sharing of electrons. Metallic character is important because of the high proportion of metals in the periodic table and the large role they play in our lives. Of the roughly 110 elements, 87 are metals. We examine each of these periodic properties individually in this section. 

Summarizing Electron Affinity for Main-Group Elements  

On the blank periodic table in the margin, locate the following  

Ionization energy is the energy change for the removal of an electron. Let s consider the energy change associated with the addition of an electron. The thermochemical equation for the addition of an electron to a fluorine atom is 

We have defined electron affinity to reflect the tendency for a neutral atom to gain an electron. An alternative definition refers to the energy change in the 

Electron affinities of some of the main group elements 

For most groups, the atom of the second row has a lower electron affinity (AgaH is less negative) than the atom of the third row. Why is this The atomic orbitals of the second row atoms are much smaller (more compact) than those of third row atoms. Consequently, when an electron is added to a second row atom, it is likely that the incoming electron encounters strong repulsive forces from other electrons in the atom and is not as tightly bound as we might otherwise expect. 

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S = Heat of sublimation of sodium D = Dissociation energy of chlorine / = Ionization energy of sodium = Electron affinity of chlorine Uq = Lattice energy of sodium chloride AHf = Heat of formation of sodium chloride. 

Electron affinities may be estimated using a Born-Haber cycle. 

Fig. V-14. Energy level diagram and energy scales for an n-type semiconductor pho-toelectrochemical cell Eg, band gap E, electron affinity work function Vb, band bending Vh, Helmholtz layer potential drop 0ei. electrolyte work function U/b, flat-band potential. (See Section V-9 for discussion of some of these quantities. (From Ref. 181.)...

Figure A3.5.4. The 488 mn photoelectron spectrum of NaClT The arrow marks the origin band, for transitions from NaCr (v = 0) to NaCl (v = 0), from which the electron affinity of NaCl is obtained.

NaCC) = 2.497 A, and = 0.195 cnV Finally, the position of the origin peak gives the electron binding energy (the electron affinity of NaCl, 0.727 eV) and a themiochemical cycle allows one to calculate the bond... 

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]... 

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... 

Because NH has an electron affinity of 0.4 eV, the total energies of the above two states can be equal only if the kinetic energy KE carried away by the ejected electron obeys... 

So, within the limitations of the single-detenninant, frozen-orbital model, the ionization potentials (IPs) and electron affinities (EAs) are given as the negative of the occupied and virtual spin-orbital energies, respectively. This statement is referred to as Koopmans theorem [47] it is used extensively in quantum chemical calculations as a means for estimating IPs and EAs and often yields results drat are qualitatively correct (i.e., 0.5 eV). 

Simons J 1973 Theory of electron affinities of small molecules J. Chem. Phys. 58 4899-907... 

The spherical shell model can only account for tire major shell closings. For open shell clusters, ellipsoidal distortions occur [47], leading to subshell closings which account for the fine stmctures in figure C1.1.2(a ). The electron shell model is one of tire most successful models emerging from cluster physics. The electron shell effects are observed in many physical properties of tire simple metal clusters, including tlieir ionization potentials, electron affinities, polarizabilities and collective excitations [34]. 

Figure Cl. 1.3 shows a plot of tire chemical reactivity of small Fe, Co and Ni clusters witli FI2 as a function of size (full curves) [53]. The reactivity changes by several orders of magnitudes simply by changing tire cluster size by one atom. Botli geometrical and electronic arguments have been put fortli to explain such reactivity changes. It is found tliat tire reactivity correlates witli tire difference between tire ionization potential (IP) and tire electron affinity...

There is a great number of mostly covalent and tetraliedral binary IV-IV, III-V, II-VI and I-VII semiconductors. Most crystallize in tire zincblende stmcture, but some prefer tire wairtzite stmcture, notably GaN [H, 12]. Wlrile tire bonding in all of tliese compounds (and tlieir alloys) is mostly covalent, some ionic character is always present because of tire difference in electron affinity of tire constituent atoms. 

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

Typical elements in Groups V. VI and VII would be expected to achieve a noble gas configuration more easily by gaining electrons rather than losing them. Electron affinity is a measure of the energy change when an atom accepts an extra electron. It is difficult to measure directly and this has only been achieved in a few cases more often it is obtained from enthalpy cycle calculations (p. 74). 

Atomic number Element Atomic radius (g) (nm) Radius ofX ion (nm) Electron affinity (kJ mol )... 

Atomic number Element Electron affinity kJ mol ) Total... 

Table 2.6 shows the electron affinities, for the addition of one electron to elements in Periods 2 and 3. Energy is evolved by many atoms when they accept electrons. In the cases in which energy is absorbed it will be noted that the new electron enters either a previously unoccupied orbital or a half-filled orbital thus in beryllium or magnesium the new electron enters the p orbital, and in nitrogen electron-pairing in the p orbitals is necessary. 

Tables 2.1, 2.2, 2.3 and 2.4 give data for atomic radii, ionisation energies and electron affinities which allow these rough rules to be justified.

AI14 electron affinity of chlorine, x 2 (two ions are formed) —728 A/15 calculated lattice energy —2539... 

The electronic configuration of each halogen is one electron less than that of a noble gas, and it is not surprising therefore, that all the halogens can accept electrons to form X" ions. Indeed, the reactions X(g) + e - X (g), are all exothermic and the values (see Table 11.1), though small relative to the ionisation energies, are all larger than the electron affinity of any other atom. 

One surprising physical property of fluorine is its electron affinity which, at — 333 kJmoPS is lower than that of chlorine, -364 kJmol , indicating that the reaction X(g) + -> X (g) is more... 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

Bromine has a lower electron affinity and electrode potential than chlorine but is still a very reactive element. It combines violently with alkali metals and reacts spontaneously with phosphorus, arsenic and antimony. When heated it reacts with many other elements, including gold, but it does not attack platinum, and silver forms a protective film of silver bromide. Because of the strong oxidising properties, bromine, like fluorine and chlorine, tends to form compounds with the electropositive element in a high oxidation state. 

In this equation, the electronegativity of an atom is related to its ionization potential, 1, and its electron affinity, E. Mulhken already pointed out that in this definition the ionization potential, and the electron affinity, E, of valence states have to be used. This idea was further elaborated by Hinze et al. [30, 31], who introduced the concept of orbital electronegativity. 

Values for these coefficients, a, b, c, of Eq. (12) can be obtained from the ionization potentials and electron affinities of the neutral, the cationic, and the anionic states of an orbital. 

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