The formation of $d\pi - p\pi$ bonds is determined by the availability of $d$-orbitals in the valence shell of the central atom. According to the sources, second-period elements such as Carbon ($C$) and Nitrogen ($N$) have only four valence orbitals ($2s$ and $2p$) and lack $d$-orbitals . As a result, these elements primarily form $p\pi - p\pi$ multiple bonds with themselves or other second-period elements like Oxygen . This is the case for $CO_{3}^{2-}$, $NO_{3}^{-}$, and $NO_{2}^{-}$. However, third-period elements like Phosphorus ($P$) have vacant $3d$ orbitals available for bonding in addition to their $s$ and $p$ orbitals . In the phosphate ion ($PO_{4}^{3-}$), the central phosphorus atom can expand its octet and use its $d$-orbitals to overlap with the $p$-orbitals of oxygen, resulting in the presence of $d\pi - p\pi$ bonds.