Let say Na2SO4... this is an example of a compound that causes a shift. sodium sulfate is a soluble salt that dissolved in water to produces the ion Na+1 and SO4-. the sulfate ion is the common ion that will cause the original equation to shift to the left.
Mass is most concentrated in the nucelus of an atom. Therefore, if you are looking to find the area with the least mass, go outside of the nucelus. Points one and four are the furthest outside of the nucleus.