Question

Why sulphur trioxide is more stable than sulphur dioxide?

Answer 1

because the valency of the sulphur is 3 and in sulphur dioxide there are only 2 molecules of oxygen so it is less stable

Answer 2

ulfur forms an expanded octet. That means that it doesn't really obey the octet rule, allowing it to take on extra electrons. Sulfur is a 3rd-period element; hence it can use its 3d orbitals to make more than 4 bonds.