To begin with, we need to draw the Lewis structures for selenium trioxide (SeO3). In total, there are three resonance structures possible for SeO3 that obey the octet rule.
1. **Structure 1:** In this resonance structure, the selenium atom is in the center, surrounded by three oxygen atoms, each connected by a double bond. The structure would be:
Se = O
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O – O
Each oxygen atom achieves an octet with the two shared electrons from the double bond, while selenium has a total of six electrons from the three double bonds.
2. **Structure 2:** In this alternative resonance structure, one of the oxygen atoms forms a double bond with selenium while the other two are single bonds, with each of those oxygens carrying a negative charge:
O = Se – O–
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O–
This structure shows one oxygen atom with a double bond and the others with single bonds, balancing the charge to satisfy the octet rule.
3. **Structure 3:** The final resonance structure has a different arrangement of double and single bonds:
O– – Se = O
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O–
Here, two of the oxygen atoms are single bonded with negative charges, while one is double bonded with the central selenium.
In all these structures, selenium is able to expand its octet, accommodating more than 8 electrons in its valence shell. The actual structure of SeO3 is a hybrid of these resonance forms, which leads to the conclusion that each oxygen atom, on average, forms 1.5 covalent bonds with the central selenium atom.
In summary, SeO3 predominantly features resonance structures with varying degrees of bond types between selenium and oxygen, demonstrating the flexibility in bonding characteristics consistent with the octet rule.