The concept of electronegativity was introduced by Linus Pauling in 1932 who defined the new atomic property as:

"The power of an atom in a molecule to attract electrons to itself."

The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativey of Atoms. Journal of the American Chemical Society Volume 54, p. 3570-3582 September 1932. The full original paper as HTML (not pdf) is available here.

Crucially, Pauling was able to quantify – put numbers to – his new parameter. While not too much should be read into absolute values, many trends in structure and reactivity behaviour can be mapped to ("explained in terms of", correlated with) Pauling's electronegativity data. This makes electronegativity an extraordinarily useful concept.

Note that hydrogen has been moved from a position above lithium to above and between boron and carbon. This is because the C–H bond is polarised ^d+C–H^d– and the B–H bond is polarised ^d+C–H^d–. IUPAC are considering the position of hydrogen on their official periodic table, here.

Where Do The Numbers Come From?

Pauling's empirical electronegativity scale is derived from thermochemical bond-energy data. Pauling observed that bond enthalpy, EA-B, in kcal/mol between atoms A and B can be predicted using the equation, where cA and cB: are the electronegativity values of A and B.

In his book The Nature of The Chemical Bond, Pauling comments that it is more accurate to use the geometric mean rather than the arithmetic mean, but then uses the arithmetic mean himself. Other authors note this and then also use the arithmetic mean.

Calculations for the formation of the halogen halides: HF, HCl, HBr & HI from hydrogen, H2, and the halogens, F2, Cl2, Br2 & I2 show how the Pauling relationship compares with experimental data:

Download the Excel spreadsheet here. Data is from Pauling's Nature of the Chemical Bond. Note that the equation requires data to be in kcal/mol rather than kJ/mol.

In Modern Inorganic Chemistry, William Jolly comments that from this approximate relationship, a useful and general principle can be derived:

The electronegativity difference between elements A and B is determined from the following relationships:

Note that both the geometric and arithmetic mean relationships are given.
For many metals the EM-M is unknown and, instead, the enthalpy of salt formation data is used as a proxy.

Once a set of electronegativity differences are known, it is a simple matter to assign absolute electronegativity values.

Compounds & Materials, Structure & Reactivity

Chlorine, by way of example, is the third most electronegative element after fluorine and oxygen. This electronegative nature is apparent in the structure and reaction chemistry of:

• The chlorine atom, Cl^•
• The dichlorine molecule, Cl2
• Ionic sodium chloride, NaCl
• Molecular chloromethane, CH3Cl
• etc.

• Electronegativity can be used to predict the dipole moment (bond polarity) of a bond:

• Electronegativity can be used to approximately predict the degree of ionic (and therefore covalent) character of a bond between two dissimilar elements:

Captured from The Nature of The Chemical Bond, 3rd Ed, pp99. The experimental values are from vapour phase dipole moments.

• Electronegativity can be used to predict metallic, ionic, covalent and intermediate bond type, and these behaviours can be mapped to the Van Arkel-Ketelaar Triangle of Bonding, as discussed in detail on the next page of the Chemogenesis web book, here.
  
  
• When valency is included as an additional parameter, electronegativity can be mapped to the Laing Tetrahedron of Bonding & Material Type, as discussed on the next but one page of the Chemogenesis web book, here.
  
  
• Electronegativity can be used to predict chemical reactivity because: "The most stable arrangement of [polar] covalent bonds connecting a group of atoms is that arrangement in which the atom with the highest electronegativity be bonded to the atom with the lowest electronegativity." Jolly, Modern Inorganic Chemistry, McGraw-Hill (1985) pp 61-62.
  
  It follows that pairs of compounds of the type A-Bm and X-Yn will react with each other to maximise and minimise electronegativty difference, as discussed the page: Why Do Chemical Reactions Happen?, here.
  
  
• Electronegativity, along with bond-length, pKa and other data, is central to the chemogenesis analysis, as discussed in the sections of this web book: Quantifying Congeneric Behaviour and Congeneric Array Interactions, here and here.

Electronegativity and Theory

Pauling used bond enthalpy data to construct his electronegativity scale. Other workers have used other starting points.

More in: H.B. Michaelson, IBM J. Res. Develop. 22 1 (1978). Review article by H. O. Pritchard and H. A. Skinner: The Concept Of Electronegativity, Chem. Rev.; 1955; 55(4) pp 745 - 786.

Electronegativity seems to integrate – average – a number of arcane atomic electronic parameters. It is a proxy parameter that in a rather simple way maps to chemical structure and reactivity.

In his 1992 paper (J.Am.Chem.Soc., (1992), 114, 1510), Allen argued that configuration energy, CE, is a fundamental atomic property and is the "missing third dimension to the periodic table". He further stated that electronegativity is an 'ad hoc' parameter.

More usefully – in this author's judgement – Allen's work shows that configuration energy, CE, correlates with electronegativity.

Indeed, electronegativity is so important that in this author's judgement it should be considered to be a basic atomic property rather than a simple atomic property, here.

Tables of Data

Various scientists, including Pauling himself, have attempted to improve the original 1932 data: