Question

We write oxygen as ${\mathrm{O}}_2$ because but why not we write phosphorus as ${\mathrm{P}}_3$ while we write it as ${\mathrm{P}}_4$?

Answer 1

Oxygen existence as ${\mathrm{O}}_2$:

• The atomic size of oxygen is very small.
• Oxygen can form $\mathrm{\pi p}-\mathrm{\pi p}$ bonds because its atomic orbitals are small and significant merging is possible.
• Two covalent bonds can join two oxygen atoms together.
• Hence ${\mathrm{O}}_2$ is formed
• 

Phosphorus existence as ${\mathrm{P}}_4$ not ${\mathrm{P}}_3$:

• $\mathrm{P}$ has a high atomic size and a low proclivity to form triple bonds.
• Phosphorus cannot form $\mathrm{\pi p}-\mathrm{\pi p}$ bonds because its atomic orbitals are so vast that significant merging is impossible.
• Hence no ${\mathrm{P}}_3$ is formed
• A sigma bond connects the $\mathrm{P}$ atom with three other $\mathrm{P}$ atoms to produce the tetra-atomic molecule${\mathrm{P}}_4$.