Electronegativity

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Electronegativity is a measure of the attraction that an atom has for 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.

Pauling Scale

The Pauling Scale is a widely used, empirical scale of electronegativity values, originally developed by Linus Pauling in 1932. On this scale, the most electronegative element (fluorine) is given a value of 4.0, while the least electronegative element (francium) is given a value of 0.7. The other elements are assigned electronegativity values in between. The elements in the second period of the periodic table usually have rounded values :

• Li : 1.0
• Be : 1.5
• B : 2.0
• C : 2.5
• N : 3.0
• O : 3.5
• F : 4.0

Mulliken Scale

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.

Electronegativity Trends

Each element has a characteristic electronegativity ranging from 0 to 4 on the Pauling scale. A strongly electronegative element, like fluorine, has an electronegativity of 4 while weakly electronegative elements, such as lithium, have values close to 1. Electronegativity trends are to higher values for elements in the top right of the periodic table. Bonds between atoms with a large electronegativity difference (greater than or equal to 2.0 on the Pauling scale) are usually considered to be ionic, while values between 2.0 and 0.4 are considered polar covalent. Values below 0.4 are considered non-polar covalent bonds.

Electronegativity decreases down the periodic table and increases across, as shown below. Additionally, atomic radius decreases across but ionization energy increase.