To determine the direction of polarity of each bond, we must know the electronegativities of each atom involved in the bonds.
Si = 1.90 P = 2.19 S = 2.58
As we move right across a row in the periodic table, the atoms become more electronegative. The direction of polarity in a bond will have the partial positive charge on the less electronegative atom and the partial negative charge on the more electronegative atom. Therefore, the direction of polarity of each bond is as follows:
(δ⁺)Si - P(δ⁻) (δ⁺)Si - S(δ⁻) (δ⁻)S - P(δ⁺)
Since silicon is the least electronegative, it will have the partial positive charge in each bond. And since sulfur is the most electronegative, it will have a partial negative charge when bonded to either silicon or phosphorus.
