[Win,odd Dhamnekar]

Why does white phosphorus consist of tetrahedral P₄ molecules while nitrogen consists of diatomic N₂ molecules?

[Corribus]

It is more difficult for main group elements to form pi bonds as you move down the periodic table. This is because the constituent out-of-plane p orbitals that form pi bonds are more diffuse as n increases (3p vs 2p, say), meaning that they don't overlap as well - and bonding strength is largely determined by orbital overlap. For the same reason, compare the standard state of sulfur versus oxygen.