bond+polarity

A chemical bond can be classified into one of three types:
 * nonpolar covalent
 * polar covalent
 * polar ionic

The classification system is based on the difference in electronegativity values between the two atoms in the bond.

To identify the bond type, subtract the electronegativity values for the two atoms (higher value minus lower value).
 * If the difference is zero, the bond is classified as nonpolar covalent.
 * Example 1: the O-O bond in an oxygen molecule is nonpolar covalent because 3.5 - 3.5 = 0.
 * Example 2: the C-S bond in carbon disulfide is nonpolar because 2.5 - 2.5 = 0.
 * If the difference is greater than zero but less than 1.7, the bond is polar covalent. The term "polar" means unequal sharing of electrons.
 * Example: the C-O bond in carbon monoxide is polar covalent: 3.5 - 2.5 = 1.0.
 * If the difference is 1.7 or greater, the bond is polar ionic. The term "ionic" means the electrons are no longer being shared.
 * Example: the Na-Cl bond in sodium chloride is polar ionic because 3.0 - 0.9 = 2.1.

The degree of polarity plays an important role in the physical properties of a molecule. These physical properties include whether the substance is a solid, liquid, or gas at room temperature, and what its melting and boiling points will be.

A general rule of thumb is that as bond polarity increases, so do the melting and boiling points.

The main reason for this is that a polar bond has "poles" - one atom has more electrons than the other. This means one atom is more negatively charged (and the other is more positively charged), which turns the bond into something like a little magnet.

Polarity is shown using little "+" or "-" symbols, or using polarity arrows:



And just like magnets, if these bonds align with opposite poles, they will stick together (called intermolecular force). The more they stick together, the harder it is to separate them (e.g. melt or boil).



Here are a few applications of bond polarity, and a few visuals of bond polarity.

__Water__

Water, or dihydrogen monoxide, has the formula H 2 O and contains two O-H bonds.

These O-H bonds have an electronegativity difference of 3.5 - 2.1 = 1.4, which is a highly polar covalent bond. This makes the water molecule polar.

Because water is so polar, its boiling point is extremely high: 100 degrees C! See the picture above that shows water molecules and their bond polarities (called hydrogen bonding).

__Methane__

Methane is carbon tetrahydride, or CH 4. It is similar in weight to water, but this is where the similarities end.

Its C-H bonds are weakly polar: 2.5 - 2.1 = 0.4, meaning the molecule is weakly polar.

Methane is a gas at room temperature (unlike water) and must be cooled to -162 degrees C to get it to liquefy!

__Sodium Chloride__

Otherwise known as table salt, the Na-Cl electronegativity difference is 2.1. This 'bond' is polar ionic, meaning it is extremely polar, so much so that the sodium ion is positively charged and the chloride ion is negatively charged, and there is no electron-sharing between the ions.

Sodium chloride is a solid at room temperature, and has a melting point of a whopping 801 degrees C!



Here is a practice worksheet on polarity and its effects on physical properties: