Draw the most important Lewis structure for SeF4 and then answer the following questions. The underlined atom is the central atom. All other atoms are bonded directly to the central atom. (a) What is the electron-group geometry, according to VSEPR theory? (b) What is the molecular geometry? (c) Is this species polar or nonpolar? Hints:
The electron-group geometry refers to the spatial arrangement of the electron pairs around the central atom.
The molecular geometry refers to the spatial arrangement of the bonded atoms around the central atom.
To analyze the molecule SeF₄ (selenium tetrafluoride), we need to first draw its Lewis structure and then apply VSEPR (Valence Shell Electron Pair Repulsion) theory to determine its geometry and polarity.
The Correct Answer and Explanation is:
Step 1: Lewis Structure of SeF₄
- Count valence electrons:
- Se (Group 16) = 6 valence electrons
- F (Group 17) = 7 valence electrons × 4 = 28
- Total = 6 + 28 = 34 valence electrons
- Form bonds:
- Connect Se to each F with a single bond. Each Se–F bond uses 2 electrons × 4 = 8 electrons.
- Distribute remaining electrons:
- 34 – 8 = 26 electrons remaining.
- Each F needs 6 more electrons (3 lone pairs) × 4 = 24 electrons.
- 2 electrons remain, which go on Se as a lone pair.
- Se has:
- 4 bonding pairs (from F)
- 1 lone pair
So, the Lewis structure has:
- Se in the center
- Four F atoms around it (with 3 lone pairs each)
- One lone pair on Se
Step 2: Determine Geometries Using VSEPR Theory
(a) Electron-Group Geometry:
Se has 5 electron groups (4 bonding, 1 lone pair) → Trigonal Bipyramidal
(b) Molecular Geometry:
With 1 lone pair on a trigonal bipyramidal arrangement, the shape becomes See-Saw
(c) Polarity:
- The lone pair creates asymmetry
- The bond dipoles do not cancel
→ SeF₄ is a polar molecule
Explanation (300+ words):
Selenium tetrafluoride (SeF₄) is a classic example of how lone pairs influence molecular shape. Starting from the Lewis structure, selenium (Se) is the central atom surrounded by four fluorine (F) atoms. Fluorine forms single bonds with Se, and each fluorine holds three lone pairs. After forming these four bonds, selenium still has two electrons left, which form a lone pair. This gives Se a total of five electron domains—four bonding and one lone pair.
According to VSEPR theory, molecules adopt a shape that minimizes electron-pair repulsion. Five electron groups arrange themselves in a trigonal bipyramidal geometry to be as far apart as possible. This geometry includes three positions in a plane (equatorial) and two above and below that plane (axial). Lone pairs prefer the equatorial positions due to less repulsion.
With one lone pair in an equatorial position and four bonded fluorine atoms occupying the remaining positions, the resulting molecular geometry is called see-saw. This shape is asymmetrical due to the lone pair, which affects both the shape and electron distribution.
Because fluorine is highly electronegative, each Se–F bond is polar. However, due to the asymmetrical shape, the dipoles do not cancel out. As a result, the molecule has a net dipole moment, making SeF₄ a polar molecule.
This molecular behavior influences SeF₄’s physical and chemical properties, such as solubility and reactivity. Polarity affects interactions with other molecules, including solvation and hydrogen bonding potential.
Final Answers:
- (a) Electron-group geometry: Trigonal Bipyramidal
- (b) Molecular geometry: See-Saw
- (c) Polarity: Polar molecule