Why is Sulfur trioxide a neutral electrophile?
Sulfur is bonded to three oxygens, twice each. The sulfur in the middle technically has a formal charge of zero, but the oxygens it is bonded to are extremely electronegative (they're electron hogs), and so the sulfur atom in the middle has a partial plus charge on it. Ergo, it will be willing to accept electrons to try to compensate.