If the bonding electrons are not shared equally in a covalent bond, then the bond will be polar. To quantify how polar a bond will be we introduce the concept of Electronegativity.
Electronegativity - the ability of an atom in a molecule to attract electrons to itself.
For example, in the HF molecule, the bonding electrons spend more time on the fluorine than the hydrogen because the fluorine has a higher electronegativity than hydrogen. We indicate this slight excess of negative charge on the fluorine with the symbol δ-:
As a result of this unequal sharing of bonding electrons the HF molecule will have an electric dipole moment. The electric dipole moment is a vector quantity, and is represented by the symbol μ. In the case of HF, μ will lie along the direction of the H-F bond:
The strength of the electric dipole moment across a bond will be proportional to the difference in electronegativity of the two atoms forming the bond. There are a number of ways to quantify atom electronegativities. Below are a few numbers based on an approach by Linus Pauling.
|Li: 1.0||Be: 1.5||B: 2.0||C: 2.5||N: 3.0||O:3.5||F: 4.0|
|Na: 0.9||Mg: 1.2||Al: 1.5||Si: 1.8||P: 2.1||S: 2.5||Cl: 3.0|
The general trend of Electronegativity in the periodic table is
Chemisty, The Central Science, 10th Ed.
8.7, 8.9, 8.11, 8.29, 8.35, 8.37, 8.39
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