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Electronegativity is a measure of the attraction that an atom has for the bonding pair of electrons in a covalent bond.
Two scales of electronegativity are in common use: the Pauling scale (proposed in 1932) and the Mulliken scale (proposed in 1934). There is also a Allred-Rochow scale.
The Pauling scale was devised in 1932. On this scale, the most electronegative element (fluorine) is given an electronegativity value of 4.0; the least electronegative element (francium) has a value of 0.7, and the remaining elements have values in between. The elements in the second period of the periodic table are sometimes given rounded values (to make them easier to remember):
On the Pauling scale, hydrogen is arbitrarily assigned a value of 2.1 or 2.2.
On the Mulliken scale, numbers are obtained by averaging ionization potential and electron affinity. Consequently, the Mulliken electronegativities are expressed directly in energy units, usually electron volts. It was proposed by Robert S. Mulliken in 1934.
Each element has a characteristic electronegativity ranging from 0 to 4 on the Pauling scale. The most strongly electronegative element, fluorine, has an electronegativity of 4 while weakly electronegative elements, such as lithium, have values close to 1. The least electronegative element is caesium at 0.7. Electronegativity trends are to higher values for elements in the top right of the periodic table. The δEN indicates the ionic character of a bond. Bonds between atoms with a large electronegativity difference (greater than or equal to 1.7 on the Pauling scale) are usually considered to be ionic, while values between 1.7 and 0.4 are considered polar covalent. Values below 0.4 are considered non-polar covalent bonds. Electronegativity differences of 0 indicate a completely non-polar covalent bond.
Electronegativity decreases down the periodic table and increases across, as shown below. Additionally, atomic radius decreases across but ionization energy increase.