Use VSEPR theory to predict the electron-pair geometry and the molecular geometry of xenon tetrafluoride, XeF4.
Select one:
a. The electron-pair geometry is octahedral, the molecular geometry is square-planar.
b. The electron-pair geometry is tetrahedral, the molecular geometry is octahedral.
c. The electron-pair geometry is square-planar, the molecular geometry is tetrahedral.
d. The electron-pair geometry is tetrahedral, the molecular geometry is tetrahedral.
e. The electron-pair geometry is octahedral, the molecular geometry is tetrahedral.
The correct answer and explanation is :
The correct answer is:
a. The electron-pair geometry is octahedral, the molecular geometry is square-planar.
Detailed Explanation (300+ words):
To solve this question using Valence Shell Electron Pair Repulsion (VSEPR) theory, we first need to determine the Lewis structure of xenon tetrafluoride (XeF₄):
- Step 1: Find the number of valence electrons:
- Xenon (Xe) is a noble gas in group 18 and has 8 valence electrons.
- Each fluorine (F) atom is in group 17 and has 7 valence electrons.
- Since there are four fluorine atoms, total fluorine electrons = 4 × 7 = 28 electrons.
- Total = 8 (Xe) + 28 (F) = 36 valence electrons.
- Step 2: Arrange the atoms:
- Xenon is the central atom, and four fluorines are placed around it.
- Step 3: Form bonds:
- Each Xe–F bond uses 2 electrons.
- 4 bonds × 2 electrons = 8 electrons used for bonding.
- Step 4: Distribute remaining electrons:
- 36 total electrons – 8 bonding electrons = 28 electrons left.
- Each F atom needs 6 electrons (3 lone pairs) to complete its octet: 4 × 6 = 24 electrons used for fluorines.
- Remaining electrons: 28 – 24 = 4 electrons, which are placed on xenon as two lone pairs.
- Step 5: Predict electron-pair and molecular geometry:
- Xenon has 4 bonding pairs (to fluorines) and 2 lone pairs.
- In VSEPR theory, 6 regions of electron density (4 bonds + 2 lone pairs) around the central atom form an octahedral electron-pair geometry.
- However, only the positions of the atoms determine the molecular geometry.
- Because the two lone pairs occupy opposite positions (to minimize repulsion), the molecular shape becomes square planar—the four fluorine atoms are at the corners of a square, and lone pairs are above and below the plane.
Final Summary:
- Electron-pair geometry: Octahedral (because there are 6 regions of electron density).
- Molecular geometry: Square planar (because 2 of the 6 regions are lone pairs, leaving 4 fluorine atoms in a square plane).
Thus, option (a) is the correct choice.