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Compare the electronegativity of S and F. You need to look up the table either in your chemistry book of online. Sulfur has an electronegativity of 2.58 and Flourine has an electronegativity of 3.98. This is an indication of their affinity for electrons. Flourine attracts a lot more of the electron density than does Sulfur. However the electrons are pulled toward the 4 flourines in directions (it is a tetrahedral molecule) which do not result in a net negative direction. The molecule is non polar. If it were polar there would be a clear positive side of the molecule and and a clear negative side of the molecule due to electrons being pulled toward a specific direction.
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