Lithium fluoride ionic bonding

The ionic bond is the most obvious sort of electrostatic attraction between positive and negative charges. It is typified by cohesion in sodium chloride. Other alkali halides (such as lithium fluoride), oxides (magnesia, alumina) and components of cement (hydrated carbonates and oxides) are wholly or partly held together by ionic bonds. 

The lithium fluoride bond is highly ionic in character because of the large difference in ionization energies of lithium and fluorine. Consequently, gaseous lithium fluoride has an unusually high electric dipole. 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Now use Coulomb s law to compare the strengths of the ionic bonds in crystals of magnesium oxide and lithium fluoride. The sizes of the four ions are taken from the tabulation of radii of cations and anions in Table 5-4. 

The ionic bond, results from a transfer of electrons from one atom to another. For example, consider the compound lithium fluoride. The lithium atom has two electrons in its inner shell and one electron in its outer shell. The loss of one electron from the outer shell would leave the lithium atom with only an inner shell with its maximum of two electrons. 

The numerical values for these quantities have been extracted and summarized in Table V. These results did not surprise us, since they were predicted by ionic model calculations (19) as well as one ab initio Hartree-Fock calculation for lithium fluoride (20) (a subsequent one is also shown in Table V) which treated both monomer and dimer. However, the trend is opposite to that observed with metal and noble gas dimers, whose I.P. s are lower than the corresponding monomers. It is simply a consequence of the relative bonding strengths of the two units in the neutral and ionic forms. Alakll halide dimers are more stable as neutrals metal and noble gas dimers are generally more stable as ions. 

The ionic bond results from transfer of electrons, as, for example, in the formation of lithium fluoride. A lithium atom has two electrons in its inner shell... 

Figure 9.6 compares the formation of sodium chloride with the formation of lithium fluoride and potassium bromide. For each of these salts, the AENs are equal to or greater than 2.0. Like sodium chloride, both lithium fluoride and potassium bromide are considered mostly ionic compounds. Notice that the two atoms in each bond are well separated from each other on the periodic table. 

The bonding in molecules in which there is an almost complete electron transfer is described as ionic. An example of such an ionic diatomic molecule is lithium fluoride, LiF. To a good approximation, the bond in LiF is represented as Li+F. The energy required to completely separate the ions in a diatomic ionic molecule (Fig. 2-21) is given by the following expression ... 

Draw diagrams to represent the bonding in each of the following ionic compounds a potassium fluoride (KF) b lithium chloride (LiCI) c magnesium fluoride (MgF2) d calcium oxide (CaO). 

The elements of the second period are the least metallic in each group, and only lithium forms an ionic fluoride, LiF. Beryllium fluoride, BeF, exists in the solid state as chains of tetrahedrally coordinated beryllium atoms, with each pair of beryllium atoms bridged by two fluorine atoms, as shown in Figure 7.1 The chain structure may be thought of in terms of each beryllium atom forming two electron-pair bonds with two fluorine atoms, and then acquiring two more electron pairs as two more fluorine atoms (bonded to the two adjacent Be atoms) use their lone pairs of electrons to produce two coordinate bonds. In this way, the beryllium atom acquires its share of an octet of electrons associated with maximum stability. 

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