Electronegativity


 * Electronegativity** is defined as the ability of an atom in a molecule to attract electrons toward itself. This chemical property is used to estimate if a bond will be nonpolar covalent, polar covalent, or ionic. There is no exact calculation to determine electronegativity, but there are various methods to find an estimate, the most popular methods being the Pauling Scale and Mulliken Electronegativity.


 * Pauling Scale**

The most common method for estimating electronegativity was discovered by American quantum chemist Linus Pauling in 1932. While developing the valence bond theory, he proposed the idea of finding electronegativity through thermochemical data. Pauling created a scale which showed the amount of electronegativity in the elements of the periodic table. The scale ranges from fluorine which has the highest electronegativity, 4.0 to caesium which has a level of 0.7 (lowest electronegativity).

Trends in the Periodic Table Generally, the amount of electronegativity increases from left to right as it goes across a period. The amount decreases as electronegativity travels down a group from top to bottom. However, there are some exceptions to this rule.
 * Mulliken Electronegativity**

Another method for estimating the ability of an atom to attract electrons to itself was found by American physicist Robert Sanderson Mulliken. He proposed that the electronegativity of an atom was directly related to ionization energy and electron affinity. Ionization energy is how strongly an atom holds onto electrons, while electron affinity is defined as how strongly an atom attracts additional electrons. Therefore, if an atom has a low electron affinity and a high ionization energy it can be determined that the atom has a high electronegativity. A flaw in the Mulliken method is that the electronegativity cannot be calculated without the atom' s electron affinity. As of the year 2006, the electron affinity has only been discovered for fifty-seven elements.



Citations: http://www.webelements.com/webelements/properties/text/image-intensity/electroneg-mulliken.html http://www.britannica.com/ebc/art-63/Electronegativity-values-of-the-elements http://en.wikipedia.org/wiki/Electronegativity#Pauling_electronegativity //Chemistry: The Central Science//