a) Describe the molecule xenon trioxide, XeO3, using four possible Lewis structures, one each with zero, one, two, or three Xe-O double bonds. (b) Do any of these resonance structures satisfy the octet rule for every atom in the molecule? (c) Do any of the four Lewis structures have multiple resonance structures? If so, how many resonance structures do you find? (d) Which of the Lewis structures in (a) yields the most favorable formal charges for the molecule?
The Correct Answer and Explanation is:
) Lewis Structures for Xenon Trioxide (XeO₃):
Xenon trioxide (XeO₃) has a total of 24 valence electrons (8 from Xe and 16 from the 3 O atoms). The four possible Lewis structures can be described based on the number of Xe–O double bonds:
- Zero double bonds (All single bonds): This structure involves all single bonds between Xe and O atoms, leaving lone pairs on the O atoms. The Xe atom would have more than 8 electrons in its valence shell, as xenon is an exception to the octet rule and can expand its octet. This structure is unstable as it leads to formal charges on the atoms.
- One double bond: In this structure, one of the Xe–O bonds is a double bond, while the other two are single bonds. The Xe atom still has an expanded octet, but this configuration starts to reduce formal charges. The oxygen atoms with single bonds will each have lone pairs, and one oxygen will have a formal charge of -1.
- Two double bonds: Here, two of the Xe–O bonds are double bonds, with the third being a single bond. This arrangement places more electrons around Xe but also makes it better bonded to the oxygens, reducing formal charges. The oxygen with a single bond will have a formal charge of -1.
- Three double bonds: In this structure, all three Xe–O bonds are double bonds. This is a highly stable configuration, with no lone pairs on the oxygen atoms, and it minimizes formal charges.
(b) The Octet Rule and Satisfying the Rule:
The octet rule is satisfied for the oxygen atoms in all four possible structures. Each oxygen atom can either have two single bonds or one double bond with xenon, ensuring it has 8 electrons in its valence shell. The xenon atom, however, does not strictly follow the octet rule in these structures. For the structures with double bonds, xenon can accommodate more than 8 electrons in its valence shell, which is allowed because xenon is a noble gas and can expand its octet due to its access to d orbitals.
(c) Resonance Structures:
The resonance structures for the four Lewis structures are as follows:
- Zero double bonds: This structure does not have resonance.
- One double bond: The location of the double bond can switch between any of the three oxygen atoms, so there are 3 resonance structures.
- Two double bonds: There are 3 possible resonance structures, as the placement of the double bonds can switch between the oxygen atoms.
- Three double bonds: There is only one resonance structure because all bonds are identical.
(d) Most Favorable Formal Charges:
The most favorable formal charge distribution is achieved in the structure with three double bonds. In this structure, all oxygen atoms have a formal charge of 0, and xenon has a formal charge of 0. This distribution is the most stable and minimizes formal charges across the molecule, leading to a more stable configuration.
