Here's why:
* Elemental phosphorus is found in two main forms: white phosphorus and red phosphorus.
* White phosphorus is a highly reactive, waxy solid that exists as P₄ molecules. These molecules have a tetrahedral structure with each phosphorus atom bonded to three other phosphorus atoms.
* Red phosphorus is a less reactive form that exists as a complex network of phosphorus atoms, but it's still ultimately derived from the P₄ units.
Therefore, when discussing the atomicity of phosphorus, we generally refer to the P₄ molecule, giving it an atomicity of 4.
