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Algernon Jackson 2l wrote:Why is phosphate an exception to the octet rule?
It isn't that phosphate itself is an exception to the octet rule, but rather that the phosphorus in the phosphate acts as an exception to the octet rule to create the compound. This is because the phosphorus has access to the 3d orbital, and so it can take on more bonds (in the case of phosphate, 5 instead of the 3 expected by its group) because it is able to store more than 8 electrons in its valence shell, which is called an expanded octet.
Phosphate can have an expanded octet because it can accommodate more than 8 valence electrons. Atoms in periods 3 or greater have d orbitals in valence shell that can accommodate additional electrons. An example is phosphorus pentachloride (PCl5). In this molecule P has an expanded octet and bonds with 5 Cl.
Phosphate can have an extended octet because it is the first row of p block before the introduction of the 3d block. Therefore, it is open to be used. Hence, that is why phosphorus and elements in the p block below the 3 p are able to open and sue the 3d block. Also, it is low energy so it's not too taxing for stability.
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