What are the two things that determine whether a molecule is polar or nonpolar?
First, to determine if a given individual bond is polar, you need to know the electronegativity of two atoms involved in that bond. To find the electronegativities of all the elements, look at the periodic table (follow the link below this answer under Web Links). If the electronegativity of the two atoms has a difference of 0.3 or less, then the bond is non-polar. If the electronegativity difference is greater that 0.3 but less than 1.7, then the bond is polar.
Excuse the part about the link there, was helping someone else and that got in there some how.
I agree with most of what Kai has written but there is one other point that needs to be made. After the individual bonds are checked to see if there is a polar bond within the molecule, the next thing to check is the three-dimensional structure. A molecule can have polar bonds but still be non-polar as a molecule. For example, CO2 has two C=O polar bonds; however, it is a linear molecule and the polarities cancel out which makes the molecule as a whole non-polar. CCl4 is another one. The C-Cl bonds are polar but the molecule is tetrahedral and the individual polarities cancel so the molecule is non-polar. CHCl3 is polar but CCl4 is not for that very reason. The secret is to know the electronegativities as well as the three-dimensional shape of the molecule. Symmetry makes the molecule non-polar.
The polarity of a molecule is determined by two factors: the molecular geometry and the polarity of its individual bonds.
1. Molecular Geometry: To determine the molecular geometry, you need to know the arrangement of atoms in the molecule. This involves identifying the central atom, the surrounding atoms (called ligands), and the lone pairs of electrons around the central atom. It is important to remember that molecular geometry is influenced by the repulsion between electron pairs, which follows the VSEPR (Valence Shell Electron Pair Repulsion) theory.
2. Polarity of Individual Bonds: The polarity of a bond is determined by the difference in electronegativity between the atoms involved. Electronegativity is a measure of an atom's ability to attract shared electrons in a covalent bond. If the electronegativity difference between two atoms is significant, the bond is polar, meaning there is an uneven distribution of electron density within the bond.
Once you have determined the molecular geometry and the polarity of individual bonds, you can determine the overall polarity of the molecule. If the molecular geometry results in a symmetrical distribution of polar bonds (with equal bond dipoles canceling each other out), the molecule will be nonpolar. On the other hand, if the molecular geometry leads to an unsymmetrical distribution of polar bonds, the molecule will be polar.
Remember, to determine electronegativity, you can refer to the periodic table, which assigns each element with a corresponding value. For example, hydrogen (H) has an electronegativity of 2.20, while fluorine (F) has an electronegativity of 3.98. The difference between these values helps determine the polarity of the H-F bond.