Final answer:
The selenate ion (SeO₄²-) has a total of four resonance structures. Each structure features one double bond with selenium and different oxygen atoms carrying negative charges. Formal charges in these structures help show the electron distribution across the ion.
Step-by-step explanation:
The selenate ion, which is an analog of the sulfate ion, has the chemical formula SeO₄²-. When drawing resonance structures for selenate, we need to take into account the octet rule, the need for charge minimization, and the formal charges on each atom. Selenate has a central selenium atom surrounded by four oxygen atoms. Since selenium is in the same group as sulfur, it may also exceed the octet rule.
For the SeO₄²- ion, the resonance structures involve one selenium atom double-bonded to one of the oxygen atoms and single bonds to the other three oxygens, with two of those oxygens each carrying a negative charge to account for the ion's overall -2 charge. All the oxygen atoms have full octets, and selenium has an expanded octet. These structures can be rotated to show that any of the oxygens can be the double-bonded one, resulting in equivalent structures.
You can have a total of four resonance structures for the selenate ion, each of them showing a different oxygen atom double-bonded to the selenium atom. The formal charges are computed based on the number of valence electrons in the isolated atom, the number of electrons in lone pairs, and the number of bonds the atom has in the structure.