Electron affinity, attachment enthalpy

The energetics of EC are determined by the electron affinity (EA) of the neutral. The EA is the negative of the enthalpy of reaction of the attachment of a zero kinetic energy electron to a neutral molecule or atom ... 

It is generally accepted that the electron affinity is positive (electron gain is exothermic) if the energy release accompanies the electron attachment. The standard enthalpy of electron gain, AegTP, at a temperature T, is related to the electron affinity,... 

ENTHALPIES OF ELECTRON ATTACHMENT (ELECTRON AFFINITIES) OF ATOMS... 

The threshold energy for the dissociative electron attachment process e" + SPg(g) = SF "(g) + F(g) has been measured directly by means of mass spectrometry ( .- ). In addition, mass spectrometric studies (5) have been reported on various electron transfer reactions which provide Independent estimates of the enthalpy for the above process. Presented below is a summary of the results derived from these measurements. Also included in the summary are results obtained from an electron-impact study ( ) of the process e + SFgCl(g) = SFg (g) + Cl(g). We assume that the fragment-ions are formed in each process with no excess or kinetic energies. As a result, the derived electron affinities represent lower limits to the true value. Other reported values of EA(SFg) include >1.465 eV (9), 3.2 eV (5) and 3.66 0.04 eV (10). The first two results are based on charge-transfer studies (5-9) while the latter value represents a direct measurement of EA using the magnetron technique (10). 

This chapter describes the connections between the one-electron configurations of the elements, the structure of the Long Form of the Periodic Table and the physical properties of the elements, namely their size, ionisation energies and electron affinities or attachment enthalpies. 

The main variations of properties of the elements that are summarised in the Periodic Table can be divided into physical and chemical properties. These will be briefly described with the elements restricted to the first four rows of the Periodic Table in order to conserve space. The three most important physical properties of the elements are their size, ionisation potential and electron affinity or attachment enthalpy each of these will be discussed briefly. 

The amount of energy required to add an electron to the lowest available empty orbital of an atom in the gaseous state is called the electron affinity or attachment enthalpy ... 

Table 3.4 Some electron affinities or attachment enthalpies of the elementsfk] mol ...

The negative value of the enthalpy change for Equation 8 when n = 0 is defined (7) as the electron affinity (EA) of the oxidized species when the oxidized and reduced species are in their ground rotational, vibrational and electronic states (0 K). At any temperature for any value of n (0, positive, or negative), the thermodynamic state functions for l uation 8 are given by aX (X = G, H, or S), and the thermochemistry of electron attachment can be defined in the ion convention ("stationary electron convention") (7). The relationship between EA and aG is given by Equation 9. A similar relationship applies for adiabatic ionization energies. 

The sign convention for electron affinity can often cause confusion for students. In this textbook, why have we referred to an enthalpy change for the attachment of an electron rather than to an electron affinity ... 

The standard electrode potential convention is a different approach from that normally taken with thermodynamic quantities related to chemical change in electron number for gas-phase molecules and ions. Ionization potentials and electron affinities are referenced to the electrostatic zero potential energy of the infinitely separated electron in a field-free vacuum." The electron itself is conventionally treated as an ideal gas (the thermal electron convention) or as a subatomic particle with no heat capacity or entropy (the ion convention). lonization/electron attachment enthalpies under the two conventions differ by 1.48kcalmoU at 298 K," while the corresponding free energies under the two conventions are only equal at 0 K and 297 K. ... 

Electron affinity (EA) For a neutral atom or molecule, equal to the energy difference between the enthalpy of formation of a neutral species and the enthalpy of formation of the negative ion of the same structure. The EA is defined as the negative of the 0 K enthalpy change for the electron attachment reaction ... 

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