Before determining if a compound is polar, you need to determine whether or not the bonds in that compound are polar. You also have to determine the molecular geometry of the bonds and any electron lone pairs.
Before talking about whether or not an entire compound is polar, take a look at what determines whether or not a bond is polar. You can then apply these rules to determine if each molecule is polar or nonpolar.
What Makes a Bond Polar?
A molecule is polar if one part of it has a partial positive charge, and the other part has a partial negative charge.
When in a bond, atoms can either share electrons (covalent) or give them up (ionic). The atom that holds the electrons closer will thus be more negatively charged than the other atom.
Electronegativity is a measure of how much a particular element wants electrons. In the Resources section you will find a periodic table which reports the electronegativity of each element. The higher this number, the more an atom of that element will "hog" the electrons in a bond. For example, fluorine is the most electronegative element.
Electronegativity values can help you determine what kind of bond exists between two atoms. Is the bond likely to be ionic or covalent? To do this, find the absolute value of the difference between the electronegativities of the two atoms. Based on this value, the following table tells you if the bond is a polar covalent bond, covalent bond or ionic bond.
between 0.4 and 1.8
Think about water. What is the electronegativity difference between atoms in water? The electronegativity difference between H (2.2) and O (3.44) is 1.24. As such, the bond is polar covalent.
Bond Polarity and Molecule Polarity
As you saw above, a bond within a molecule can be polar. What does this mean for the whole molecule?
When determining molecule polarity, all bonds must be considered. This means that the vector partial charge from each bond must be added up. If they a cancel out, then the molecule may not be polar. If there are vector components left, then the bond is polar.
In order to find the direction of these vectors, you have to examine the molecular geometry of the bonds. You can find this via valence shell electron-pair repulsion (VSEPR) theory.
The theory starts with the idea that electron pairs in the valence shell of an atom repel each other (since like charges repel). As a result, the electron pairs around an atom will orient themselves to minimize repulsive forces.
Take a look at water again. Water is bound to two hydrogens and also has two lone pair electrons. It has a tetrahedral bent shape.
To determine whether or not the molecule is polar, you have to look at the partial charge vectors on the two bonds in the molecule.
First, there are two electron pairs on the molecule, which means there will be a large negative partial charge vector in that direction.
Next, oxygen is more electronegative than hydrogen and will hog the electrons. This means that the partial charge vector on each bond will have a negative component pointing toward the oxygen.
The inward component of the vector on each bond will cancel. The portion pointing toward the oxygen will not cancel. As a result, there is a net partial negative charge toward the oxygen side of the molecule. There is also a net partial position toward the hydrogen side of the molecule.
This analysis reveals that water is a polar molecule.
What about CH4?
First, CH4 has no lone pairs since all the electrons are involved in a single bond between C and H. CH4 has a tetrahedral molecular geometry.
Next, the C-H bond is covalent as the difference in electronegativities is 0.35. All the bonds are covalent, and there will not be a big dipole moment. Thus, CH4 is a nonpolar molecule.
The difference between polar and nonpolar molecules can thus be found by the vectors of partial charge resulting from each bond.
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