In the Lewis structure of PO43- (phosphate ion), the presence of a double bond is due to the need to satisfy the octet rule for the central phosphorus atom. Phosphorus, being in group 15 of the periodic table, has five valence electrons. In the phosphate ion, phosphorus is surrounded by four oxygen atoms.
To achieve a stable octet, phosphorus forms a double bond with one of the oxygen atoms. This double bond allows phosphorus to share two additional electrons, helping it to reach a total of eight electrons in its valence shell. The remaining three oxygen atoms form single bonds with phosphorus, and each of these oxygen atoms also carries a negative charge, contributing to the overall 3- charge of the ion.
Here is a simplified representation of the Lewis structure:
O
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O - P - O
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O-In this structure, the double bond between phosphorus and one oxygen atom ensures that phosphorus has a complete octet, while the single bonds and lone pairs on the oxygen atoms help to distribute the negative charge across the ion.