Sulfur trioxide (SO3) is a nonpolar molecule. This is because of its symmetrical trigonal planar shape. In SO3, the sulfur atom is at the center with three oxygen atoms evenly spaced around it. The bond dipoles of the three S-O bonds cancel each other out due to this symmetry, resulting in no net dipole moment.
Additionally, the electronegativity difference between sulfur and oxygen is not large enough to create a significant dipole moment. Therefore, SO3 is considered a nonpolar molecule.