Electronegativity and Bond Polarity
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.
| H: 2.1 | |||||||
| 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 | S: 1.8 | P: 2.1 | S: 2.5 | Cl: 3.0 |
The general trend of Electronegativity in the periodic table is
Using the Electronegativities we can predict whether a given bond will be non-polar, polar covalent, or ionic. The greater the difference in electronegativity the more polar the bond.
Order the following bonds according to polarity: H-H, O-H, Cl-H, S-H, and F-H.
In practice no bond is totally ionic. There will always be a small amount of electron sharing.
So what is and ionic compound? There's no clear cut line. Therefore we use the following practical definition for an ionic compound
Ionic Compound is any solid that conducts an electric current when melted or dissolved in water.
Or you can simply say that a "salt" is an ionic compound.
Once again, the picture we're forming is that, in virtually every case, the atom in a stable compound has a noble gas arrangment of electrons. When two non-metals react to form a covalent bond, they share electrons in a way that completes the valence electron configurations of both atoms (i.e., both non-metals attain a noble gas configuration).
Homework from Chemisty, The Central Science, 10th
Ed.
8.7, 8.9, 8.11, 8.29, 8.35, 8.37, 8.39