Volcanoes produce a variety of molten substances, including sulfur and silicon dioxide - Scottish Highers Chemistry - Question 1 - 2015

A possible structure for P₄S₃ is:

      S
    /   \
   P - P
  /       \
 S         P
  \       /
   S - P

The covalent radius of sulfur is smaller than that of phosphorus due to the increased nuclear attraction for electrons. Since sulfur has a higher nuclear charge with more protons in its nucleus compared to phosphorus, it pulls the electron cloud closer, resulting in a smaller covalent radius.

The melting point of sulfur is significantly higher than that of phosphorus due to the types of intermolecular forces present. Sulfur atoms are held together by London dispersion forces, which are weaker than the stronger covalent bonds in phosphorus molecules.

Moreover, sulfur has more atoms in its molecular structure (S₈) compared to phosphorus (P₄), leading to a greater surface area for London dispersion forces to act upon in sulfur.

Due to these stronger forces in sulfur compared to the weak forces between phosphorus molecules, sulfur's melting point is much higher.