Halides elemental

Peroxide formation may intervene [50] during dehydroxylation of hydroxides of the following metals Ba, Al, Cd, Zn and Pb. This conclusion was reached from the observation that during the reactions of these salts with alkali halides elemental halogen was liberated. These reactions need further investigation. 

Fig. 1.1 Periodic table showing the elements in the major groups and transition metals. Only the elements associated with life are listed. The common elements are in dark green. Light orange/yellow or pink white backgrounds indicate the metal and halide elements that occur as ions. Trace elements are indicated in red, pink, or light green (Modified from the full Periodic Table found in Wikipedia at http //en.wikipedia.org/wiki/Periodic table)...

The element fluorine has nine protons, ten neutrons, and nine electrons, giving a mass of 19 Da. Fluorine is the smallest halide element (Sect. 1.1.5) and it forms a gas that is so electron deficient that it has all reacted with metals and only its ions (F 1) are found on earth. Fluoride and hydroxide ions have almost the same mass, 19 Da and 17 Da respectively. Because fluoride is more electronegative, it replaces the hydroxide ion on crystals without altering their overall structure. This type of alteration is referred to as an isomor-phous ion replacement. 

Belloni J.,Treguer M., Remita H., De Keyzer R., Enhanced yield of photoinduced electrons in doped silver halide crystals. Nature, 1999,402, 865-867. De Keyzer R., Treguer M., Belloni J., Remita H., Photosensitive silver halide element with increased photosensitivity, US patent 6,436,625, Aug. 20,2002. 

The Use ofComplexing Halides ( Elements to be Nitrided and Alkaline Metals 233... 

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

For the formation of the hydrogen halides by the direct combination of the elements, the enthalpies of formation are ... 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

The tribromide and triodide of both boron and aluminium can be made by the direct combination of the elements although better methods are known for each halide. The properties of each halide closely resemble that of the chloride. 

CHLORIDES AND OTHER IMPORTANT HALIDES OF GROUP IV ELEMENTS... 

All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

Nitrogen does form a number of binary compounds with the halogens but none of these can be prepared by the direct combination of the elements and they are dealt with below (p. 249). The other Group V elements all form halides by direct combination. 

The rigid classification of halides into covalent and ionic can only be an oversimplification, and the properties of the halides of a given element can very greatly depend upon the halogen. Thus the classification is only one of convenience. 

Anhydrous halides, however, are obtained when the metal is heated with the dry hydrogen halide or the halogen. In the case of elements with more than one oxidation state, the hydrogen halide produces a lower halide and the halogen a higher halide, for example... 

Halides of non-metals are usually prepared by the direct combination of the elements. If the element exhibits more than one oxidation state, excess of the halogen favours the formation of the higher halide whilst excess of the element favours the formation of the lower halide (e.g. PCI5 and PCI3). 

These are halides formed by highly electropositive elements (for example those of Groups I and II, except for beryllium and lithium). They have ionic lattices, are non-volatile solids, and conduct when molten they are usually soluble in polar solvents in which they produce conducting solutions, indicating the presence of ions. 

When the several halides of a given element are considered, changes in bond character are also found. The fluoride is generally the most ionic with ionic character decreasing from fluoride to... 

When an element has more than one oxidation state the lower halides tend to be ionic whilst the higher ones are covalent—the anhydrous chlorides of lead are a good example, for whilst leadfll) chloride, PbCl2, is a white non-volatile solid, soluble in water without hydrolysis, leadflV) chloride, PbC, is a liquid at room temperature (p. 200) and is immediately hydrolysed. This change of bonding with oxidation state follows from the rules given on p.49... 

These are formed by less electropositive elements. They are characterised by the existence of discrete molecules which exist even in the solid state. They have generally lower melting and boiling points than the ionic halides, are more volatile and dissolve in non-polar solvents. 

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