- Draw Lewis structure for SF4
(sulfur tetrafluoride), determine its point group, and list all symmetry elements (9 points).
The Correct Answer and Explanation is:
Lewis Structure for SF₄ (Sulfur Tetrafluoride)
Sulfur tetrafluoride (SF₄) consists of a central sulfur (S) atom surrounded by four fluorine (F) atoms. To draw the Lewis structure, follow these steps:
- Count the valence electrons:
- Sulfur (S) is in Group 16, so it has 6 valence electrons.
- Each fluorine (F) atom is in Group 17, so it has 7 valence electrons.
- Total valence electrons = 6 (for S) + 4 × 7 (for F) = 34 electrons.
- Place sulfur in the center: Sulfur is less electronegative than fluorine, so it will be in the center.
- Connect the fluorine atoms to sulfur: Use single bonds to connect each fluorine atom to sulfur.
- Complete the octet of fluorine atoms: Each fluorine atom needs 6 more electrons to complete its octet, so place 3 lone pairs of electrons around each fluorine.
- Distribute remaining electrons on sulfur: Sulfur has 2 remaining electrons. These will be placed as lone pairs on sulfur. Since sulfur can have an expanded octet, it can hold more than 8 electrons.
- Final Structure:
- Sulfur is bonded to four fluorine atoms by single bonds.
- Each fluorine has 3 lone pairs.
- Sulfur has 2 lone pairs.
The structure looks like a seesaw shape, due to one lone pair on the sulfur atom.
Point Group of SF₄
SF₄ is classified under the C₃v point group.
Symmetry Elements:
- E (Identity operation): The structure is invariant under no operation, meaning the molecule remains unchanged.
- C₃ (Rotation axis): SF₄ has a 3-fold rotational axis passing through the sulfur atom and the two fluorine atoms in the plane. Rotating by 120° or 240° around this axis does not change the structure.
- σₓ (Mirror plane): There are two mirror planes in SF₄:
- One plane containing the sulfur atom and the two opposite fluorine atoms (plane of symmetry).
- Another plane that contains sulfur and one fluorine atom, bisecting the angle between the other two fluorines.
- σᵧ (Mirror plane): A plane that includes the C₃ axis (same as above).
- S₃ (Improper rotation): A combination of a 120° rotation about the C₃ axis followed by a reflection in the mirror plane leaves the molecule unchanged.
Explanation:
SF₄ is a trigonal bipyramidal molecule with one lone pair on sulfur, resulting in a seesaw-shaped geometry. This shape explains its symmetry, including the presence of a 3-fold rotation axis (C₃) and mirror planes (σ). The symmetry elements describe how the molecule behaves under certain operations, which is important for predicting its spectroscopic properties and behavior in molecular interactions.
