Electronegativity

Key points: The electronegativity of an element is the power of an atom of the element to attract electrons when it is part of a compound; there is a general increase in electronegativity across a period and a general decrease down a group.

The electronegativity, (chi), of an element is the power of an atom of the element to at tract electrons to itself when it is part of a compound. If an atom has a strong tendency to acquire electrons, it is said to be highly electronegative (like the elements close to fluorine). Electronegativity is a very useful concept in chemistry and has numerous applications, which include a rationalization of bond energies and the types of reactions that substances undergo and the prediction of the polarities of bonds and molecules. Periodic trends in electronegativity can be related to the size of the atoms and electron configuration. If an atom is small and has an almost closed shell of electrons, then it is more likely to attract an electron to itself than a large atom with few valence electrons. Consequently, the electronegativities of the elements typically increase left to right across a period and decrease down a group. Quantitative measures of electronegativity have been defined in many different ways. Linus Pauling’s original formulation (which results in the values denoted P in Table 1.7) draws on concepts relating to the energetics of bond formation, A definition more in the spirit of this chapter, in the sense that it is based on the properties of individual atoms, was proposed by Robert Mulliken. He observed that, if an atom has a high ionization energy, I, and a high electron affinity, Ea , then it will be likely to acquire rather than lose electrons when it is part of a compound, and hence be classified as highly electronegative. Conversely, if its ionization energy and electron affinity are both low, then the atom will tend to lose electrons rather than gain them, and hence be classified as electropositive. These observations motivate the definition of the Mulliken electronegativity, M , as the average value of the ionization energy and the electron affinity of the element (both expressed in electronvolts):

Table 1.7Pauling X_P, Mulliken, X_M, and Allred-Rochow, X_AR, electronegativities

The hidden complication in the apparently simple definition of the Mulliken electron egativity is that the ionization energy and electron affinity in the definition relate to the valence state, the electron configuration the atom is supposed to have when it is part of a molecule. Hence, some calculation is required because the ionization energy and electron affinity to be used in calculating M are mixtures of values for various actual spectroscopically observable states of the atom. We need not go into the calculation, but the resulting values given in Table 1.7 may be compared with the Pauling values (Fig. 1.27). The two

Figure 1.27 The periodic variation of Pauling electronegativities.

scales give similar values and show the same trends. One reasonably reliable conversion between the two is

Because the elements near F (other than the noble gases) have high ionization energies and appreciable electron affinities, these elements have the highest Mulliken electronegativities. Because M depends on atomic energy levels—and in particular on the location of the highest filled and lowest empty orbitals—the electronegativity of an element is high if the two frontier orbitals of its atoms are low in energy.Various alternative ‘atomic’ definitions of electronegativity have been proposed. A widely used scale, suggested by A.L. Allred and E. Rochow, is based on the view that electronegativity is determined by the electric field at the surface of an atom. As we have seen, an electron in an atom experiences an effective nuclear charge Z_eff . The Coulombic potential at the surface of such an atom is proportional to Z_eff /r, and the electric field there is proportional to Z_eff /r². In the Allred–Rochow definition of electronegativity, X_AR is assumed to be proportional to this field, with r taken to be the covalent radius of the atom:

The numerical constants have been chosen to give values comparable to Pauling electro-negativities. According to the Allred–Rochow definition, elements with high electro negativity are those with high effective nuclear charge and the small covalent radius: such elements lie close to F. The Allred–Rochow values parallel closely those of the Pauling electronegativities and are useful for discussing the electron distributions in compounds.

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Polyatomic molecules

Homonuclear diatomic molecules

The hydrogen molecule

The basic shapes

Resonance

Polarizability

Electron affinity

Atomic and ionic radii

The format of the periodic table

The periodic table

Penetration and shielding

The angular variation of atomic orbitals