The placement of the atoms does not matter. Remember, these molecules are NOT flat. They are three-dimensional. This particular molecule is tetrahedral. If you put the F’s across from each other, they will not cancel one another out, as there would still be a net DOWNWARD AND INTO THE PAPER pull by the F’s and a net UPWARD AND OUT OF THE PAPER pull by the Cl’s. In a tetrahedral geometry, the only way to get bonds to cancel each other is for all bonds to be identical. If even one bond is different, the pull will be unbalanced, because the bonds are not 90 degrees relative to each other. They are 109 degrees relative to each other.