Whether or not a molecule is polar depends entirely upon the polarity of the bonds found in a given compound and some parameters of these bonds. But before delving into how to to determine polarity, here is a quick explanation of polarity
What Makes Something Polar?
A molecule is polar if one part of it has a partial positive charge, and another part has a partial negative charge.
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.
Electronegativity values can help you determine if a bond between two atoms is likely to be covalent or polar covalent. In order to do this, you find the absolute value of the difference between the electronegativities of the two atoms. Based on this difference, the following table tells you if the bond is polar covalent, covalent or ionic.
between 0.4 and 1.8
For example, since the electronegativity difference between H (2.2) and O (3.44) is 1.24, this bond would be polar covalent. But what does that mean for a molecule containing an O-H bond?
Bond Polarity vs. Molecule Polarity
While a bond may be polar within a molecule, the molecule itself may not. Why is this?
Partial charges or dipole moments (resulting from bond polarity) are important in determining molecular polarity. But, all bonds must be considered. If the vectors of partial charge/dipole moment end up canceling out, then the molecule may not be polar.
In order to predict dipole moments, you have to examine the geometry of the bonds which you can find via valence shell electron-pair repulsion (VSEPR) theory. This theory starts with the idea that electron pairs in the valence shell of an atom repel each other. The electron pairs around an atom will thus orient themselves in order to minimize repulsive forces.
Take a look at water. Water is bound to two hydrogens and also has two lone pairs of electrons. Because of the two loan pairs, the molecule has a tetrahedral bent shape. In order to determine whether or not the molecule is polar, you have to look at the partial charge vectors.
First, there are two electron pairs on the molecule, which means there will be a large 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 component pointing toward the oxygen.
While the inward component of the vector on each bond will cancel, the portion pointing towards the oxygen will not. As such, there will be a net partial negative charge on the oxygen side of the molecule and net partial position on the hydrogen side of the molecule. Thus, water is a polar molecule.
What about CO2?
First, CO2 has no lone pairs since all the electrons are involved in two sets of double bonds between C and O. This means that CO2 has a linear geometry.
Next, the C-O bond is polar covalent as the difference in electronegativities is 0.89. Now, you need to map the dipole moment to do the molecular geometry. One end of the molecule has a partial negative charge pointing out toward the oxygen. But this is also true on the other end. As a result, the dipole moments cancel out.
Thus, CO2 is a nonpolar molecule.
Test yourself: Is CH4 polar or nonpolar?
Hint: Draw out the molecular shape, and then calculate the electronegativity difference.
Answer: Since all the dipole moments cancel in this tetrahedral molecule, CH4 is nonpolar.