Part A. In a Lewis structure, all of the atoms except hydrogen have a complete octet; that is, each atom (except hydrogen) has eight electrons (the octet rule). However, there are a few structures that break this rule by having 10 or even 12 valence electrons around a central atom. If the central atom has more than eight valence electrons, this means that the central atom has expanded its valence shell. An expanded valence shell minimizes the formal charge on some of the atoms.
Draw a single Lewis structure for the phosphate ion (PO43− ), assuming that the phosphorus atom can expand its octet such that the formal charge on each of the atoms is minimized. Include all formal charges and lone-pair electrons.
Draw the molecule by placing atoms on the grid and connecting them with bonds. Include all lone pairs of electrons and formal charges. To add a formal charge, click or and then click on an atom.
For example, consider the two Lewis structures for the sulfate ion (SO42−):
In the normal octet structure, sulfur has a +2 formal charge and all four oxygen atoms have a − 1 formal charge. Because sulfur belongs to the third period, it shows an expanded octet that minimizes the formal charge on each of the atoms. Thus in the expanded Lewis structure for a sulfate ion, the sulfur atom has 12 valence electrons, and the formal charge on the sulfur atom gets reduced to 0 by forming two sulfur-oxygen double bonds. The formal charge on two oxygen atoms also gets reduced to 0.
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