The four molecular elements

Phosphorus, sulfur, chlorine and argon are simple molecular substances with only van der Waals attractions between the molecules. Their melting or boiling points are lower than those of the first four members of the period which have complex structures. The magnitudes of the melting and boiling points are governed entirely by the sizes of the molecules, which are shown again for reference:

• Phosphorus: Elemental phosphorus adopts the tetrahedral  P₄ arrangement. Melting phosphorus breaks no covalent bonds; instead, it disrupts the much weaker van der Waals forces between the molecules.
• Sulfur:  Elemental sulfur forms S₈ rings of atoms. The molecules are bigger than phosphorus molecules, and thus the van der Waals attractions are stronger, leading to a higher melting and boiling point.
• Chlorine: Chlorine, Cl₂, is a much smaller molecule with comparatively weak van der Waals attractions, and thus chlorine will have a lower melting and boiling point than sulfur or phosphorus.
• Argon: Elemental argon is monatomic . The scope for van der Waals attractions between argon atoms is very limited and so the melting and boiling points of argon are lower again.