Diatomic molecules electronegativity

FIGURE 3.33 A typical d molecular orbital energy-level diagram for a heteronuclear diatomic molecule AB the relative contributions of the atomic orbitals to the molecular orbitals are represented by the relative sizes of the spheres and the horizontal position of the boxes. In this case, A is the more electronegative of the two elements. 

Values of the dipole moment of some diatomic molecules are given in Table 2.11. The SI unit of dipole moment is the coulomb-meter (C-m). This is a very large unit, so in Table 2.11 we use the unit 10-30 C-m. Dipole moments are often quoted in an older unit, the debye (D) 1 D = 3.24 X 1CT30 C-m. We can see from Table 2.11 that the dipole moment of a diatomic molecule usually reflects the difference between the electronegativities of the two atoms. 

Boiling points, molecular weights, and electronegativity differences for some diatomic molecules. 

Atoms do not all have the same ability to attract electrons. When two different types of atoms form a covalent bond by sharing a pair of electrons, the shared pair of electrons will spend more time in the vicinity of the atom that has the greater ability to attract them. In other words, the electron pair is shared, but it is not shared equally. The ability of an atom in a molecule to attract electrons to it is expressed as the electronegativity of the atom. Earlier, for a homonuclear diatomic molecule we wrote the combination of two atomic wave functions as... 

An HC1 molecule is a heteronuclear diatomic molecule composed of H (EN = 2.1) and Cl (EN = 3.0). Because the electronegativities of the elements are different, the pull on the electrons in the covalent bond between them is unequal. Hence HC1 is a polar molecule. 

This presents a problem when we discuss the dipole moment of a polar heteronuclear diatomic molecule, AX, where X will be the more electronegative. In the chemical picmre, it is quite common to say that in the ground state the molecule lies along some axis and that it has a definite dipole moment. In the physical picmre, we say that the molecule has no measurable dipole moment in the ground state. 

Hinze et al. (1963) suggest that charge transfer in diatomic molecules obeys the principle of equality of energies involved in the process. If we adopt equation 1.31 as a definition of electronegativity and imagine that the amount of energy involved in the transfer of a fraction of charge dz from atom A to atom B is quantifiable as... 

One classical example that apphes the electroneutrality principle is the electronic structure of carbon monoxide, a diatomic molecule with a very small dipole moment of 0.110 debye. The only electronic structure that satisfies the octet rule for CO is C=0 , a structure that corresponds to C and O, if the shared electron pairs are equally devided by the two atoms. Pauling showed that the electronegativity difference of 1.0 would correspond to about 22% partial ionic character for each bond, and to charges of and 0° +. A second possible electronic structure, C=O , does not complete the octet for carbon. The partial ionic character of the bonds corresponds to C0.44+ If these two structures contribute... 

Symbol Br atomic number 35 atomic weight 79.904 a halogen group element electron affinity 3.36359 eV electronegativity 2.8 electron configuration [Ar] 3di°4s24p5 most stable valence states -1 and -i-5, less stable valence states -1-1 and -i-3 a diatomic molecule (Br2) in liquid and vapor states over a wide range of temperature two stable isotopes, Br-79 (50.57%) and Br-81 (49.43%). 

Figure 1.9 Molecular orbital diagram for the general case of a diatomic molecule AB, where B is more electronegative than A. Reprinted, by permission, from R. E. Dickerson, H. B. Gray, and G. P. Haight, Jr., Chemical Principles, 3rd ed., p. 464. Copyright 1979 by Pearson Education, Inc.

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