Iodine electron affinity

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... 

C08-0073. Repeat the calculation of Problem 8.37 for K and I, using 500 kJ/mol as the estimated second electron affinity of iodine and assuming no change in distance of closest approach. 

Figure 5.16. Plot of data for the external heavy-atom quenching of pyrene fluorescence in benzene at 20°C. Polaro-graphic half-wave reduction potentials Ein are used as a measure of the electron affinity of the quencher containing chlorine (O), bromine ( ), or iodine (3). From Thomaz and Stevens<148) with permission of W. A. Benjamin, New York.

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

Through reduction or oxidation of the molecule by a dopant molecule. Atoms or molecules with high electron affinity, such as iodine, antimony pentafluoride (SbCls), or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), may oxidize a typical organic semiconductor such as poly(p-phenylene) derivatives, leaving them positively charged. Reduction, i.e., addition of an electron, may be obtained by doping with alkali metals. 

Blandamer et al (1964) pointed out that the absorption spectrum of iodine ions (I ) in NH3 has its maximum at Jtv=4.0eV the difference from that for an electron in a cavity, 4.0-0.8= 3.2eV, corresponds well to the electron affinity of iodine. In water the maxima for both I and a solvated electron are shifted by 0.8 eV to higher frequencies we deduce that the energy of the bottom of the conduction band in water is about 0.8 eV. 

Which atom has the greatest electron affinity chlorine, bromine, iodine ... 

Unlike the metallic elements, halogens become less reactive going down the periodic table because of their generally decreasing electron affinity. Thus, their reactivity order is F2 > CI2 > Br2 > I2. Fluorine often reacts violently, chlorine and bromine somewhat less so, and iodine often sluggishly. 

The trend in oxidation potentials may be considered a composite trend, similar to that described for the E° values of the alkali metals (Chap. 6). For the halogens, the following quantities are involved heats of dissociation of the molecules, electron affinities of the atoms, hydration energies of the ions, heats of vaporization (for bromine and iodine only), and, finally, entropy or randomness effects. Aside from the entropy effects (which turn out to be quite small for the reactions being considered), the reduction of the halogen X to the hydrated ion X at room temperature may be represented in steps as follows ... 

Electron affinity tends to decrease down a group. For example, fluorine has a higher electron affinity than iodine. 

The chemistry of gold is more diversified than that of silver. Six oxidation states, from -I to III and V, occur in its chemistry. Gold(-I) and Auv have no counterparts in the chemistry of silver. Solvated electrons in liquid ammonia can reduce gold to give the Au" ion which is stable in liquid ammonia (E° = -2.15 V). In the series of binary compounds MAu (M = Na, K, Rb, Cs), the metallic character decreases from Na to Cs. CsAu is a semiconductor with the CsCl structure and is best described as an ionic compound, Cs+Au . The electron affinity of gold (—222.7 kJ mol"1) is comparable to that of iodine (-295.3 kJ mol-1). Gold in the oxidation state -I is also found in the oxides (M+ Au O2 (M = Rb, Cs) these, too, have semiconducting properties.1... 

Ionisation potentials in the group are generally high, but fall markedly with atomic number. Iodine appears as a unipositive ion in many of its reactions. Electron affinities show a maximum at chlorine. Nevertheless fluorine is the better oxidising agent in aqueous solution, and even when dry will usually replace chlorine from its compounds. 

Fluorine is, in fact, an extremely powerful oxidising agent. In spite of lower dissociation energies bromine and iodine are weaker oxiding agents than chlorine this is due to their smaller electron affinities and smaller hydration energies. 

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