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# We write oxygen as ${{O}_{2}}$ because $(10-8$(atomic no. Of oxygen)) but why not write phosphorus as $P_3(18-15$(atomic no. of phosphorus)) while we write it as $P_4$?

Last updated date: 30th Mar 2023
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Due to the large atomic size of the phosphorus, it is unable to form pi bonds and it is tetratomic in which each p atom is linked with the $3$ other p atoms by sigma bond. To form a triple bond between the two phosphorus there should be enough space in between them. As the atomic size is large it is not possible. They use d orbitals to make pi bonds.
Remember that the Hund’s maximum spin multiplicity implies only for the orbitals that have the same amount of energy. That means you need not to put electrons in the $4p$ orbital of Krypton until the $4s$ orbital is full.