Electron affinity

In chemistry and atomic physics, the electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion.

X + e⁻ → X⁻ + energy

"Electron affinity" as defined in solid state physics

Simple Band Diagram with denoted vacuum energy E_VAC, conduction band edge E_C, Fermi level E_F, valence band edge E_V, electron affinity E_ea, work function Φ and band gap E_g

In solid state physics, especially when referring to semiconductors, the electron affinity is defined differently than the chemistry definition. Electron affinity is defined instead as the energy required to move an electron from the bottom of the lowest conduction band to a stationary state in the nearby vacuum:

$E_{\rm ea} \equiv E_{\rm VAC} - E_{\rm C}$

In an intrinsic semiconductor at absolute zero, this concept is analogous to the chemistry definition of electron affinity, since typically an added electron will go into the lowest conduction band. At nonzero temperature, and for other materials (metals, semimetals, heavily doped semiconductors), the analogy does not hold since an added electron will instead go to the Fermi level on average.

The electron affinity is a similar concept to the work function, but distinct: the work function is the thermodynamic work required to reversibly, isothermally move an electron from the material to the vacuum (the thermodynamic electron comes from the Fermi level on average, not from the conduction band edge). While the work function of a semiconductor can changed by doping, the electron affinity is typically a constant for a given type of material.

Measurement and use of electron affinity

This property is measured for atoms and molecules in the gaseous state only, since in the solid or liquid states their energy levels would be changed by contact with other atoms or molecules. A list of the electron affinities was used by Robert S. Mulliken to develop an electronegativity scale for atoms, equal to the average of the electron affinity and ionization potential. Other theoretical concepts that use electron affinity include electronic chemical potential and chemical hardness. Another example, a molecule or atom that has a more positive value of electron affinity than another is often called an electron acceptor and the less positive an electron donor. Together they may undergo charge-transfer reactions.

To use electron affinities properly, it is essential to keep track of sign. For any reaction that releases energy, the change ΔE in total energy has a negative value and the reaction is called an exothermic process. Electron capture for almost all non-noble gas atoms involves the release of energy and thus are exothermic. The positive values that are listed in tables of E_ea are amounts or magnitudes. It is the word, released within the definition energy released that supplies the negative sign. Confusion arises in mistaking E_ea for a change in energy, ΔE, in which case the positive values listed in tables would be for an endo- not exo-thermic process. The relation between the two is E_ea = −ΔE(attach).
However, if the value assigned to E_ea is negative, the negative sign implies a reversal of direction, and energy is required to attach an electron. In this case, the electron capture is an endothermic process and the relationship, E_ea = −ΔE(attach) is still valid. Negative values typically arise for the capture of a second electron, but also for the nitrogen atom.

The usual expression for calculating E_ea when an electron is attached is

E_ea = (E_initial − E_final)_attach = −ΔE(attach)

This expression does follow the convention ΔX = X(final) − X(initial) since −ΔE = −(E(final) − E(initial)) = E(initial) − E(final).

Equivalently, electron affinity can also be defined as the amount of energy required to detach an electron from a singly charged negative ion, i.e. the energy change for the process

X⁻ → X + e⁻

If the same table is employed for the forward and reverse reactions, without switching signs, care must be taken to apply the correct definition to the corresponding direction, attachment (release) or detachment (require). Since almost all detachments (require +) an amount of energy listed on the table, those detachment reactions are endothermic, or ΔE(detach) > 0.

Electron affinities of the elements

EA vs Atomic Number

Main article: Electron affinity (data page)

Although E_ea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive E_ea than metals. Atoms whose anions are more stable than neutral atoms have a greater E_ea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values.

E_ea generally increases across a period (row) in the periodic table. This is caused by the filling of the valence shell of the atom; a Group 17 atom releases more energy than a Group 1 atom on gaining an electron because it obtains a filled valence shell and therefore is more stable.

A trend of decreasing E_ea going down the groups in the periodic table would be expected. The additional electron will be entering an orbital farther away from the nucleus. Since this electron is farther from the nucleus it is less attracted to the nucleus and would release less energy when added. However, a clear counterexample to this trend can be found in Group 2, and this trend only applies to Group 1 atoms. Electron affinity follows the trend of electronegativity. Fluorine (F) has a higher electron affinity than oxygen and so on.

The following data are quoted in kJ/mol. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds.

Electron affinities in the periodic table
Group →	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
↓ Period
1	H 73																	He *
2	Li 60	Be *											B 27	C 122	N *	O 141	F 328	Ne *
3	Na 53	Mg *											Al 42	Si 134	P 72	S 200	Cl 349	Ar *
4	K 48	Ca 2	Sc 18	Ti 8	V 51	Cr 65	Mn *	Fe 15	Co 64	Ni 112	Cu 119	Zn *	Ga 41	Ge 119	As 79	Se 195	Br 324	Kr *
5	Rb 47	Sr 5	Y 30	Zr 41	Nb 86	Mo 72	Tc *	Ru 101	Rh 110	Pd 54	Ag 126	Cd *	In 39	Sn 107	Sb 101	Te 190	I 295	Xe *
6	Cs 46	Ba 14	*	Hf	Ta 31	W 79	Re *	Os 104	Ir 150	Pt 205	Au 223	Hg *	Tl 36	Pb 35	Bi 91	Po	At	Rn *
7	Fr	Ra	**	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Cn	Uut	Fl	Uup	Lv	Uus	Uuo
* Lanthanides			La 45	Ce 92	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm 99	Yb	Lu 33
** Actinides			Ac	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr
Legend
The number mentioned is Electron affinity in kJ/mol (rounded)
For the equivalent value in eV, see: Electron affinity (data page)
* Denotes elements that are expected to have electron affinities close to zero on quantum mechanical grounds
Color of the atomic number shows state of matter (at 0 °C and 1 atm): black=Solid green=Liquid red=Gas grey=Unknown Border shows natural occurrence of the element: Primordial From decay Synthetic Background color shows subcategory in the metal–nonmetal range: Metal Metalloid Nonmetal Unknown chemical properties Alkali metal Alkaline earth metal Inner transition metal Transition metal Post-​transition metal Other nonmetal Halogen Noble gas Lan­thanide Actinide

Molecular electron affinities

The electron affinity of molecules is a complicated function of their electronic structure. For instance the electron affinity for benzene is negative, as is that of naphthalene, while those of anthracene, phenanthrene and pyrene are positive. In silico experiments show that the electron affinity of hexacyanobenzene surpasses that of fullerene.

Electron affinity of surfaces

The electron affinity measured from a material's surface is a function of the bulk material as well as the surface condition. Often negative electron affinity is desired to obtain efficient cathodes that can supply electrons to the vacuum with little energy loss. The observed electron yield as a function of various parameters such as bias voltage or illumination conditions can be used to describe these structures with band diagrams in which the electron affinity is one parameter. For one illustration of the apparent effect of surface termination on electron emission, see Figure 3 in Marchywka Effect.

See also

• Electron-capture mass spectrometry
• Koopmans' theorem
• One-electron reduction
• Ionization energy
• Electronegativity
• Valence electron
• Work function
• Marchywka Effect
• Vacuum level
• Electron donor

References

• Tro, Nivaldo J. (2008). Chemistry: A Molecular Approach (2nd Edn.). New Jersey: Pearson Prentice Hall. ISBN 0-13-100065-9. pp. 348–349.

External links

• Electron affinity, definition from the IUPAC Gold Book