Covalent bonds in diatomic molecules

A relationship between the bonding in diatomic molecules and in solids has been demonstrated. It follows from the so-called Universal Binding Energy Relation (UBER), applicable to both metals and covalent diatomic molecules. The energy and the interatomic separation are scaled in the following way ... 

A covalent bond, as we have said, is the sharing of an electron pair by two atoms. In a homomclear diatomic molecule like H2, in which the atoms are identical, the electrons are equally shared, that is, the electrons spend the same amount of time in the vicinity of each atom. However, in a covalently bonded heteronuclear diatomic molecule like HF, the H and F atoms do not share the bonding electrons equally because H and F are different atoms... 

We have already noted that the properties of the elements in Group VI show the characteristic trends that we have come to expect on descending a Group. The elements become more metallic in character oxygen is a covalently bonded gaseous diatomic molecule sulfur is a solid containing Sg molecules and is an insulator selenium (non-metal) and tellurium (semi-metal) are semiconductors with polymeric structures polonium is a metal. The compounds of selenium, tellurium and pollonium also illustrate the inert pair effect and a tendency to higher coordination numbers. 

In Section 2.12, we saw that a polar covalent bond in which electrons are not evenly distributed has a nonzero dipole moment. A polar molecule is a molecule with a nonzero dipole moment. All diatomic molecules are polar if their bonds are polar. An HC1 molecule, with its polar covalent bond (8+H—Clfi ), is a polar molecule. Its dipole moment of 1.1 D is typical of polar diatomic molecules (Table 3.1). All diatomic molecules that are composed of atoms of different elements are at least slightly polar. A nonpolar molecule is a molecule that has no electric dipole moment. All homonuclear diatomic molecules, diatomic molecules containing atoms of only one element, such as 02, N2, and Cl2, are nonpolar, because their bonds are nonpolar. 

It is important to point out that almost all bonds are polar bonds, whether they are approximately described as covalent or ionic. The bonds in the molecules of the various forms of the elements such as the diatomic molecules H2, CI2, and N2, larger molecules such as P4 and Sg, and infinite molecules such as diamond may be described as pure covalent bonds... 

Another type of bond is the covalent bond, in which one, two, or more pairs of electrons are shared by two or more atoms. Unlike ionic bonds, covalent bonds involve the sharing of electrons by atoms. The simplest example of covalent bonding occurs when two atoms of hydrogen bond to form a diatomic hydrogen molecule (H + H yield H H, or H ). 

For the covalent bonds in the diatomic molecules studied by Bader and Essen (1984), values of ky and k2 vary from —25 to —45 eA-s, while /3 is positive in the range of 0-45 eA-5. The sum of the curvatures, V2p, is invariably negative, indicating the concentration of electron density in the internuclear region. But for second-row atoms, the larger positive value of A3 may dominate the Laplacian. 

A shared pair of electrons creates a single covalent bond in F. Often the double dots forming the bond are replaced with a short dash to represent the bond (F-F). The other halogens form diatomic molecules with single bonds in a similar fashion. 

The weakness of the covalent bond in dilithium is understandable in terms of the low effective nuclear charge, which allows the 2s orbital to be very diffuse. The addition of an electron to the lithium atom is exothermic only to the extent of 59.8 kJ mol-1, which indicates the weakness of the attraction for the extra electron. By comparison, the exother-micity of electron attachment to the fluorine atom is 333 kJ mol-1. The diffuseness of the 2s orbital of lithium is indicated by the large bond length (267 pm) in the dilithium molecule. The metal exists in the form of a body-centred cubic lattice in which the radius of the lithium atoms is 152 pm again a very high value, indicative of the low cohesiveness of the metallic structure. The metallic lattice is preferred to the diatomic molecule as the more stable state of lithium. 

The covalent bonds in all homonuclear diatomic molecules must be nonpolar. 

Because two atoms of the same element are forming the bond, the difference in electronegativities is zero. In the fluorine molecule, a pair of valence electrons are shared equally. This type of bond is a pure covalent bond. All other diatomic elements (CI2, Br2,12, O2, N2, and H2) have pure covalent bonds. In all these molecules, the electrons are shared equally. 

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