Halogen molecule

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

In these examples tire entropy change does not vaty widely, and the value of the equilibrium constant is mainly determined by the heat of dissociation. It can be concluded, tlrerefore, that niuogen is one of the most stable diatomic molecules, and tlrat chlorine is tire most stable diatomic halogen molecule. 

The standard entropy change for the atom-molecule reactions is in the range 5-20 mole and the halogen molecule dissociation has an eiiU opy change of about 105 e.u. The halogen molecule dissociation energy decreases from chlorine to iodine, but the atom-molecule reactions become more endothermic from chlorine to iodine, and this latter effect probably influences the relative contributions to the mechanism from chain reaction and biinolecular reaction. 

The next step is to consider tire cross-sections of the absorption of radiation by the diatomic halogen molecules in order to decide if the relative effects result from the efficiency of the radiation photon-molecule interactions. These are reflected in the dissociation cross-sections of tlrese interactions. 

Initiation step (Section 4.17) A process which causes a reaction, usually a free-radical reaction, to begin but which by itself is not the principal source of products. The initiation step in the halogenation of an alkane is the dissociation of a halogen molecule to two halogen atoms. 

Figure 17.2 Schematic molecular orbital energy diagram for diatomic halogen molecules. (For F2 the order of the upper and 7T bonding MOs is inverted.).

When 4-ethynyl-3,5-dimethylpyrazole interacts with bromine, only one halogen molecule is added, probably owing to steric hindrance (71TH1) (Scheme 78). 

Table 6-VI lists some properties of the halogens. In the elemental state, the halogens form stable diatomic molecules. This stability is indicated by the fact that it takes extremely high temperatures to disrupt halogen molecules to form the monatomic species. For example, it is known that the chlorine near the surface of the sun, at a temperature near 6000°C, is present as a gas consisting of single chlorine atoms. At more normal temperatures, chlorine atoms react with each other to form molecules ...

Table 19-11. sizes of the halogen atoms, melting points, boiling points, AND DISSOCIATION PROPERTIES OF THE HALOGEN MOLECULES... 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

FIGURE 2.20 Bond lengths fin picometers) of the diatomic halogen molecules. Notice how the bond lengths increase down the group as the atomic radii become larger. 

Since the recognition of the role of chlorine in catalytic ozone destruction, increasing effort has been devoted to finding replacements. In most cases reported so far, the replacements are partially halogenated molecules that retain one or more hydrogen atoms (HCFCs and HFC s). The presence of H-atoms gives HO a handle (via H-atom abstractions such as R4) for their tropospheric... 

A number of other monomers may be employed as variations on the materials mentioned so far, to introduce specific properties into the finished resin. For example, halogenated molecules containing either chlorine or bromine atoms may be used to confer fire resistance. As described in Chapter 8, the effect of halogens in the polymer structure is to make the resins difficult to ignite and unable to sustain combustion. 

The molecules in crystalline chlorine, bromine and iodine are packed in a different manner, as shown in Fig. 11.1. The rather different distances between atoms of adjacent molecules are remarkable. If we take the van der Waals distance, such as observed in organic and inorganic molecular compounds, as reference, then some of the intermolecular contacts in the b-c plane are shorter, whereas they are longer to the molecules of the next plane. We thus observe a certain degree of association of the halogen molecules within the b-c plane (dotted in Fig. 11.1, top left). This association increases from chlorine to iodine. The weaker attractive forces between the planes show up in the plate-like habit of the crystals and in their easy cleavage parallel to the layers. Similar association tendencies are also observed for the heavier elements of the fifth and sixth main groups. 

Another very useful photolytic homolysis is that of halogen molecules to yield atoms,... 

Homolysis of halogen molecules heating or irradiation with light of a wave length that can be absorbed by the halogen molecules. 

F2 has the most positive standard reduction potential and therefore is the strongest of all common oxidizing agents. Oxidizing strengths of the diatomic halogen molecules decrease down Group 7A. 

A halogen molecule contains one nonpolar covalent single bond, as shown by its Lewis structure. As we descend the 7A family from F2 to I2, the size of a halogen atom increases and so the bond length increases. The strength of the X-X bond varies it increases from F2 to Cl2, then decreases from Cl2 to Br2 to I2. 

M. Giihr, in Coherent Dynamics of Halogen Molecules in Rare Gas Solids, ed. by S. De Silvestri, G. Cerullo, G. Lanzani. Coherent Vibrational Dynamics (CRC, Boca Raton, 2007), p. 173... 

Bonding found within halogen molecule is covalent — formed as a result of sharing of electrons. 

Forces found between halogen molecules are van der Waals forces, which are due to temporarily induced dipoles caused by polarization of electron clouds. 

---