Draw the Lewis Structure for the following molecules, including any resonance structures and formal charges. The central atom is bolded and italicized: a)
b)
c)
d)
State the total valence electron count, VSEPR molecular geometry, and hybridization of the central atom for each molecule. Determine whether the molecule has a net electronic dipole (is it polar?).
The Correct Answer and Explanation is:
Here is the detailed analysis for each molecule as requested.
Analysis of Molecules
a) BF₃ (Boron trifluoride)
- Total Valence Electrons: 3 (from B) + 3 × 7 (from F) = 24
- Lewis Structure: Boron is the central atom, forming single bonds with three fluorine atoms. Each fluorine has three lone pairs. This structure has zero formal charges on all atoms. While resonance structures can be drawn giving boron a full octet, they place a positive formal charge on the highly electronegative fluorine atom, making them minor contributors. The dominant structure is the one with an incomplete octet on boron.
:F: | :F — B — F:content_copydownloadUse code - VSEPR Molecular Geometry: The central boron atom has 3 electron domains (3 single bonds, 0 lone pairs). This corresponds to an AX₃ arrangement, resulting in a Trigonal Planar geometry.
- Hybridization of Central Atom: 3 electron domains require sp² hybridization.
- Polarity: The B-F bonds are polar. However, the molecule’s symmetrical trigonal planar shape causes the individual bond dipoles to cancel each other out perfectly. Therefore, the molecule is nonpolar.
b) OSCl₂ (Thionyl chloride)
- Total Valence Electrons: 6 (from O) + 6 (from S) + 2 × 7 (from Cl) = 26
- Lewis Structure: Sulfur is the central atom. The most stable Lewis structure features a double bond between sulfur and oxygen, single bonds between sulfur and each chlorine, and one lone pair on the sulfur atom. This arrangement minimizes formal charges to zero for all atoms.
:O: || :Cl — S — Cl: ..content_copydownloadUse code with caution. - VSEPR Molecular Geometry: The central sulfur atom has 4 electron domains (1 double bond, 2 single bonds, 1 lone pair). This corresponds to an AX₃E₁ arrangement, resulting in a Trigonal Pyramidal geometry.
- Hybridization of Central Atom: 4 electron domains require sp³ hybridization.
- Polarity: The S-O and S-Cl bonds are polar, and the trigonal pyramidal geometry is asymmetrical. The lone pair and the bond dipoles do not cancel, creating a net dipole moment. The molecule is polar.
c) SCN⁻ (Thiocyanate ion)
- Total Valence Electrons: 6 (from S) + 4 (from C) + 5 (from N) + 1 (for the charge) = 16
- Lewis Structure and Resonance: Carbon is the central atom. The ion exists as a resonance hybrid of three structures. The most significant contributor places a double bond between S and C, a double bond between C and N, and the negative formal charge on the most electronegative atom, nitrogen. A second important contributor has a single bond between S and C, and a triple bond between C and N, placing the negative charge on sulfur.
[ :S=C=N: ]⁻ <--> [ :S—C≡N: ]⁻ <--> [ :S≡C—N: ]²⁻ FC: S=0, C=0, N=-1 FC: S=-1, C=0, N=0 (Major Contributor)content_copydownloadUse code with caution. - VSEPR Molecular Geometry: The central carbon atom has 2 electron domains in all significant resonance structures. This corresponds to an AX₂ arrangement, resulting in a Linear geometry.
- Hybridization of Central Atom: 2 electron domains require sp hybridization.
- Polarity: As an ion, it is charged. The charge distribution within the ion is uneven because sulfur and nitrogen have different electronegativities. The bond dipoles do not cancel, so the ion possesses a permanent dipole moment and is considered polar.
d) H₂CCl₂ (Dichloromethane)
- Total Valence Electrons: 2 × 1 (from H) + 4 (from C) + 2 × 7 (from Cl) = 20
- Lewis Structure: Carbon is the central atom, single-bonded to two hydrogen atoms and two chlorine atoms. Each chlorine atom has three lone pairs.
H | :Cl — C — Cl: | Hcontent_copydownloadUse code with caution. - VSEPR Molecular Geometry: The central carbon atom has 4 electron domains (4 single bonds, 0 lone pairs). This corresponds to an AX₄ arrangement, resulting in a Tetrahedral geometry.
- Hybridization of Central Atom: 4 electron domains require sp³ hybridization.
- Polarity: While the molecular geometry is tetrahedral, the atoms attached to the central carbon are not identical. The C-Cl bonds are significantly more polar than the C-H bonds. This asymmetry means the bond dipoles do not cancel out. There is a net pull of electron density toward the two chlorine atoms, creating a net dipole moment. The molecule is polar.
Explanation
Determining the properties of a molecule begins with its Lewis structure, a two-dimensional representation of valence electron distribution. The first step is to sum the valence electrons from all atoms, adjusting for any ionic charge. These electrons are then arranged to form bonds and lone pairs, aiming to satisfy the octet rule for most atoms while minimizing formal charges. Some atoms, like boron in BF₃, are stable with an incomplete octet. Others, like sulfur in OSCl₂, can accommodate an expanded octet to achieve a more stable state with zero formal charges. When multiple valid Lewis structures can be drawn by moving electrons, as in the SCN⁻ ion, the molecule is described as a resonance hybrid of these structures.
The completed Lewis structure is the key to predicting molecular shape using Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory states that electron domains (which include single bonds, multiple bonds, and lone pairs) around a central atom will arrange themselves to be as far apart as possible, minimizing repulsion. This arrangement dictates the electron geometry. The molecular geometry, which describes the position of only the atoms, is then determined by considering the effect of any lone pairs. For example, both H₂CCl₂ and OSCl₂ have four electron domains and a tetrahedral electron geometry, but the lone pair on sulfur causes OSCl₂ to have a trigonal pyramidal molecular geometry.
The number of electron domains also determines the hybridization of the central atom’s orbitals (sp, sp², sp³), which describes the mixing of atomic orbitals to form the hybrid orbitals required for bonding. Finally, molecular polarity depends on both bond polarity and molecular geometry. A molecule with polar bonds can be nonpolar if its geometry is perfectly symmetrical, causing the bond dipoles to cancel, as seen in BF₃. Conversely, an asymmetrical molecule like H₂CCl₂ or OSCl₂ will be polar because its bond dipoles result in a net electronic dipole moment.thumb_upthumb_down
