Biology Question:

A polar molecule is a molecule that has a net dipole moment due to its having unsymmetrical polar bonds.

Two factors go into determining if a molecule is a polar. To determine if a molecule (or ion) is polar or non-polar, you must determine both factors.

1) The polarity of the individual bonds in the molecule;

2) The shape or geometry of the molecule

First, to determine if a given individual bond is a polar, you need to know the electro negativity of two atoms involved in that bond. To find the electro negativities of all the elements, look at the periodic table (follow the link to the left of this answer under Web Links). If the electro negativity of the two atoms has a difference of 0.3 or less, then the bond is non-polar. If the electro negativity difference is greater that 0.3 but less than 1.7, then the bond is polar.

Once you know which bonds in the molecule are known as polar and which are non-polar, you must use the shape of the molecule. You need the shape because two polar bonds, if oriented correctly can cancel each other out (like two equally strong people pulling in opposite directions on a rope -- nobody moves). There are 3 possible outcomes: