Ionic compounds diatomic molecules

His second observation concerns the polarity of chemical compounds. Diatomic molecules like F2 or CI2 are completely nonpolar. The highest polarity is found in ionic compounds like NaF or KCl, but in between there appears to be a large number of molecules of intermediate polarity. This observation suggests that the formation of bonds is frequently accompanied by a partial transfer of electrons between the bonded atoms. A good model for the chemical bond should therefore be flexible enough to allow a gradual transition from nonpolar to ionic bonding. Lewis postulated that ... 

A solid that contains cations and anions in balanced whole-number ratios is called an ionic compound. Sodium chloride, commonly known as table salt, is a simple example. Sodium chloride can form through the vigorous chemical reaction of elemental sodium and elemental chlorine. The appearance and composition of these substances are very different, as Figure 2-24 shows. Sodium is a soft, silver-colored metal that is an array of Na atoms packed closely together. Chlorine is a faintly yellow-green toxic gas made up of diatomic, neutral CI2 molecules. When these two elements react, they form colorless ciystals of NaCl that contain Na and Cl" ions in a 1 1 ratio. 

Lewis and many other chemists had recognized the shortcomings of the ionic bond. When diatomic molecules, such as or Cl, were considered, there was no reason why one atom should lose an electron and an identical atom should gain an electron. There had to be another explanation for how diatomic molecules formed. We have seen how the octet rule applies to the formation of ionic compounds by the transfer of electrons. This rule also helps explain the formation of covalent bonds when molecules (covalent compounds) form. Covalent bonds result when atoms share electrons. Using fluorine, F, as a representative halogen, we can see how the octet rule applies to the formation of the molecule. Each fluorine atom has seven valence electrons and needs one more electron to achieve the stable octet valence configuration. If two fluorines share a pair of electrons, then the stable octet configuration is achieved ... 

The formula units He and F2 represent single atoms and diatomic molecules, respectively. Unlike the F2 molecule, in which one fluorine atom is bonded to a specific other fluorine atom, in the ionic compound NaCl, one Na ion is bonded to six Cl ions that are adjacent to it. Each of the Cl ions is bonded to six Na ions that are adjacent to it. (The fifth and sixth ions are in layers in front of and behind the layer shown here see Figure 5.9.) The ratio of Na" ions to CF ions is therefore 1 1. Any pair ofNa+ andCr ions, such as those circled in red or the one circled in green, is a formula unit. 

Diatomic molecules uncombined elements Charges on ions ionic compounds... 

There are only a few heteronuclear diatomic molecules that are formed from elements of the first and second rows of the Periodic Table and are stable as diatomic molecules in the gas phase at normal temperatures and pressures. These are HF, CO and NO. Others have been observed at high temperatures, in discharge lamps, in flames or in space. Examples are LiH, LiF, OH, BeH, BeO, BF, BH, CH, CN and NH. Some of the molecules in this second list will be stable with respect to the two separate atoms but not at normal temperatures and pressures with respect to other forms of the compound. LiH, LiF and BeO are normally found as ionic solids. The other molecules are unstable with respect to covalent compounds in which the atoms have their normal valencies H20, BeH2, BF3, B2H6, CH4, (CN)2 and NH3. 

In Group 7A(17), fluorine and chlorine have the condensed electron configuration [noble gas] ns np, as do the other halogens (Br, I, At). Little is known about rare, radioactive astatine (At), but all the others are reactive nonmetals that occur as diatomic molecules, X2 (where X represents the halogen). All form ionic compounds with metals (KX, MgX2), covalent compounds with hydrogen (HX) that yield acidic solutions in water, and covalent compounds with carbon (CX4). 

Table 6.1. Gaseous heteronuclear diatomic molecules AB(g) hydrogen halides HX interhalogen compounds XK alkali metal hydrides MH, and inter-alkali-metal compounds MM. Electric dipole moments, ionic characters, qjc equilibrium bond distances. Re, vibrational wavenumbers, < dissociation energies at zero K, Dq rednced masses of the predominant isotopomers, /tm and force constants,. ...

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