Electronegativity scale, definition

Equation [33] is Mulliken s definition of electronegativity, so the linear coefficient in the Taylor series is the electronegativity of the atom. Mulliken s definition is consistent with other electronegativity scales. The second-order coefficient, j J, is the hardness of the atom, For semiconductors, the hardness is half the band gap, and r] is an important property in inorganic and acid-base chemistry. " Physically, IP - EA is the energy required to transfer an electron from one atom to another atom of the same type,... 

X ", X "(H) has been taken as 2.2. Although eq. 2.12 implies that the units of x " are eV, it is not customary to give units to electronegativity values. By virtue of their different definitions, values of x on different electronegativity scales (see below) possess different units. 

Another fundamental property of chemical bonds is polarity. In general, it is to be expected that the distribution of the pair of electrons in a covalent bond will favor one of the two atoms. The tendency of an atom to attract electrons is called electronegativity. There are a number of different approaches to assigning electronegativity, and most are numerically scaled to a definition originally proposed by Pauling. Part A of Table 1.6... 

This was justified as follows. The energy required to take an electron from a neutral atom Y to a neutral atom Z is /Y — AZt whereas the energy cost to take an electron from a neutral atom Z to a neutral atom Y is Iz — Ay. Hence, the two atoms Y and Z would have an equal propensity for attracting electrons or equal electronegativity if /Y — Az = Iz — AY, that is if IY + Ay = Iz + Az. This is consistent with the Mulliken definition, eqn (3.35), the factor 1/2 being arbitrary. As can be seen by comparing the two different scales in Fig. 3.5, the Mulliken values are approximately 2.8 times the Pauling values. 

Redox reactions are better defined in terms of the concept of electron transfer. Thus an atom is said to be oxidized if, as the result of a reaction, it experiences a net loss of electrons and is reduced if it experiences a net gain of electrons. This simple definition can be used to identify oxidation or reduction processes at carbon in terms of a scale of oxidation states for carbon based on the electronegativities of the atoms attached to carbon. The idea is to find out whether in a given reaction carbon becomes more, or less, electron-rich. We will use the following somewhat arbitrary rules ... 

For A = B the difference in electronegativity is zero, and for A / B the difference is defined as A = xa %b = / Daa — DBb The well-known Pauling scale of electronegativities results from this definition of xa on specifying dissociation energies in units of eV. 

Both of the xp and Xm scales are empirical approximations based on incomplete experimental data. The theoretical definition of absolute electronegativity, x = a/Eg = /Xm — > Xv has been demonstrated to account for both empirical scales. The scale factor x varies with periodic shell and //, represents the number of valence energy-level vacancies. 

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