Electron affinity, halogens

Threshold energies of both reactions differ by the differences of halogen electron affinities and alkali ionization potentials. In all cases reaction (1.27a) is energetically favoured. However, it involves a transition from the ionic electronic ground state into the covalent excited electronic states which lead to atoms. Under some circumstances reaction (1.27b) may be favoured, since the electronic transition in step (1.27a) is forbidden. 

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. 

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. 

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... 

By introducing reasonable values (about 2 for nitrogen, 4 for oxygen) for the electron affinity parameter relative to carbon, 8, and for the induced electron affinity for adjacent atoms (32/8i = Vio), we have shown that the calculated permanent charge distributions for pyridine, toluene, phenyltrimethylammonium ion, nitrobenzene, benzoic acid, benzaldehyde, acetophenone, benzo-nitrile, furan, thiophene, pyrrole, aniline, and phenol can be satisfactorily correlated qualitatively with the observed positions and rates of substitution. For naphthalene and the halogen benzenes this calculation does not lead to results... 

Halogens, the elements in Group 17 of the periodic table, have the largest electron affinities of all the elements, so halogen atoms (a n readily accept electrons to produce halide anions (a a. This allows halogens to react with many metals to form binary compounds, called halides, which contain metal cations and halide anions. Examples include NaCl (chloride anion), Cap2 (fluoride anion), AgBr (bromide anion), and KI (iodide anion). 

However, no evidence for even a transitory existence of -(X-Y) has been obtained except in the cases of X = Y = halogen or CNS (ref. 575a). and it is probable that the breakdown is concerted with reduction. The mole of cleavage appears to be governed by the relative electron affinities of X- and Y-, for example, hypobromous acid and hydroxylamine are cleaved by reducing ions as follows... 

ECD Halogenated compounds Extremely sensitive to high-electron affinity compounds Ease of use Cheap Limited to ion detection Poor selectivity Small linear range [31]... 

The electron affinities for the halogens are the highest of any group of elements. 

Table 7.2 Electron Affinities and Dissociation Energies for Halogens. ...

For non-metal elements, such as the halogens (group 7), the concept of electron affinity is far more useful than ionisation energy. These elements form negative ions in ionic compounds. 

The first electron affinity of each halogen in the series chlorine to iodine is shown in the table below. 

As the size of the halogen atom increases, the force of attraction between the nucleus of the atom and the incoming electron decreases. Therefore, the first electron affinities become less exothermic down group 7. 

Symbol Br atomic number 35 atomic weight 79.904 a halogen group element electron affinity 3.36359 eV electronegativity 2.8 electron configuration [Ar] 3di°4s24p5 most stable valence states -1 and -i-5, less stable valence states -1-1 and -i-3 a diatomic molecule (Br2) in liquid and vapor states over a wide range of temperature two stable isotopes, Br-79 (50.57%) and Br-81 (49.43%). 

---