Write the Lewis structure for SeF4. What is the correct information about the electron-pair arrangement and shape of the molecule? Select one: a. (Electron-pair arrangement: AX4, SN = 4; tetrahedral.) Shape: tetrahedral. b. (Electron-pair arrangement: AX2E, SN = 3; trigonal planar.) Shape: bent. c. (Electron-pair arrangement: AX4E2, SN = 6, and octahedral.) Shape: square planar. d. (Electron-pair arrangement: AX4E, SN = 5, and trigonal bipyramidal.) Shape: seesaw. e. (Electron-pair arrangement: AX2E3, SN = 5, and trigonal bipyramidal.) Shape: linear
The Correct Answer and Explanation is:
The correct answer is:
d. (Electron-pair arrangement: AX₄E, SN = 5, and trigonal bipyramidal.) Shape: seesaw.
Explanation:
To understand the Lewis structure and molecular geometry of selenium tetrafluoride (SeF₄), we begin by determining its valence electrons and applying VSEPR (Valence Shell Electron Pair Repulsion) theory.
1. Lewis Structure:
- Selenium (Se) is in Group 16 and has 6 valence electrons.
- Each fluorine (F) atom contributes 7 valence electrons, and there are 4 fluorine atoms, giving a total of 28 electrons from fluorine.
- Total valence electrons: 6 (Se) + 28 (F) = 34 electrons.
- Selenium forms four single bonds with fluorine atoms, using 8 electrons.
- Each fluorine needs 6 more electrons to complete its octet (24 electrons in total).
- So far: 8 (Se–F bonds) + 24 (F lone pairs) = 32 electrons used.
- We have 34 – 32 = 2 electrons left. These go on the central Se atom as a lone pair.
Thus, Se has:
- 4 bonding pairs (to F)
- 1 lone pair
This gives a Steric Number (SN) of 5 → 5 regions of electron density around Se.
2. Electron-Pair Geometry:
With 5 regions of electron density, the electron-pair geometry is trigonal bipyramidal.
3. Molecular Shape (Actual Shape):
One of the positions is occupied by a lone pair, and four positions are occupied by F atoms. In a trigonal bipyramidal geometry, lone pairs prefer the equatorial position to minimize repulsion.
So, with 4 bonds and 1 lone pair:
- The molecular shape is seesaw.
Final Answer:
d. (Electron-pair arrangement: AX₄E, SN = 5, and trigonal bipyramidal.) Shape: seesaw.
