Electronegativity may be described as the power of an atom to attract electrons to itself in a chemical bond. It is the most important chemical parameter in calculating the type of chemical bonds formed in between atoms. It is hard to quantify in a satisfactory way, especially as electronegativity is not a property of atoms on their own, but relays to some extent on their state of chemical combination. Never the less several scales have been devised.

• Pauling electronegativity is based on bond energies, using the empirical observation that bonds between nuclei atoms with a large electronegativity difference tend to be stronger than those where the difference is small. This scale was historically the first to be devised and although it lacks a theoretical justification is still used.
• Mulliken electronegativity is the average of the electron affinity of an atom and first ionization energy, reflecting the importance of two possibilities in bond formation, losing an electron or gaining one. The scale has the plus point that electronegativity values can be estimated not only for the ground states of atoms, but for other electron configurations and even for polyatomic fragments.

• Allred-Rochow electronegativity is proportional to Zeff/r2, where Zeff is the nuclear charge of valence orbitals, and r the covalent radius of the atom. The value is proportional to the effective electrostatic attraction on valence electrons by the nucleus, screened by inner shell electron.

Fig. 1. Pauling electronegativity values for the elements H-K. Elements in the shaded region are metallic.

Each scale gives different numbers and they should not be mixed. The broad general trends do, agree: electronegativity decreases towards the bottom and increases towards the right in the periodic table. It thus follows the similar trend as atomic ionization energies. Elements in stating groups have low values and are called electropositive. Figure 1 describes the Pauling electronegativities of elements up to potassium. Element of group 18 in early periods do not form any stable compounds, and so the most electronegative element is fluorine.