Here's why:

* Atomic Mass: Each element has a unique atomic mass, which is the average mass of all its isotopes.

* Sulfur (S): Has an atomic mass of approximately 32.06 atomic mass units (amu).

* Oxygen (O): Has an atomic mass of approximately 16.00 amu.

* Isotopes: Both sulfur and oxygen have multiple isotopes, which are atoms of the same element with different numbers of neutrons. While isotopes of the same element have similar chemical properties, their masses differ due to the different number of neutrons.

Therefore, a sulfur atom is roughly twice as massive as an oxygen atom.