Molecular Structure & Bonding Concepts

Overview

This lecture reviews ACS practice questions focused on molecular structure, bonding, geometry, hybridization, resonance, and related key chemical concepts.

Molecular Geometry & Bond Angles

• NH3 and BF3 combine to form NH3BF3, both with tetrahedral geometry due to four electron groups around N and B.
• ML4 with four single bonds and no lone pairs has a tetrahedral shape.
• NF3 is trigonal pyramidal due to a lone pair on nitrogen.
• Ozone (O3) and SO2 are geometrically similar because their central atoms have similar valence electron configurations.
• Carbonate ion (CO3^2-) and CO2 are planar; CO3^2- is trigonal planar, and CO2 is linear.

Bond Angles & Electron Groups

• AX5E (like in some molecules) forms a square pyramidal geometry.
• XeF4 (AX4E2) has a square planar geometry due to two opposing lone pairs.
• NO2^-, NO2, and NO2^+ have increasing bond angles as the number of lone pairs on the central atom decreases (NO2^+ ≈ 180°, NO2 ≈ <120°, NO2^- < NO2).
• Linear geometry: bond angle ≈ 180°, trigonal planar ≈ 120°, tetrahedral ≈ 109.5°.
• BF2^- (beryllium difluoride) is linear with a bond angle of 180°; H2O has a bond angle of 104.5°.

Resonance, Bond Order & Structure

• SO2 has multiple resonance structures; its bond order is 1.5.
• Four equivalent covalent bonds (as in CH4) arise from sp3 hybridization.
• CO2 uses sp hybridization (linear, two electron groups).
• Largest dipole moment occurs in molecules with asymmetric, polar bonds (like bent molecules).

Polarity & Solid Structures

• Quartz is a network covalent solid, each Si bonded to four O atoms; not like CO2.
• Bond angles in OCN- are about 120°, indicating trigonal planar geometry.
• In BF3 (trigonal planar), dipoles cancel; in PF3 (trigonal pyramidal), they do not, resulting in a net dipole.

Octet Rule & Exceptions

• Compounds like SO4^2- are tetrahedral, not linear.
• NO2 (with an odd electron) is a free radical and violates the octet rule.
• Hydrogen bonding explains water's higher boiling point compared to similar molecules.

Hybridization & Bond Lengths

• Ethane (C–C single), ethene (C=C double), benzene (C–C bond order 1.5) exhibit increasing bond strength and decreasing length.
• Structure stability favored when formal charges are minimized.
• Nitrogen forms triple bonds most readily among common elements.

Sigma & Pi Bonds; Resonance

• For typical hydrocarbons: count sigma (σ) and pi (π) bonds (e.g., 6 σ, 3 π).
• Resonance: molecules with hybrid orbitals have delocalized electrons.

Key Terms & Definitions

• Tetrahedral — Geometry with four bonds, bond angles of ~109.5°.
• Trigonal pyramidal — Shape with three bonds and one lone pair, bond angle ~107°.
• Resonance — Different valid Lewis structures for the same molecule.
• Hybridization — Mixing atomic orbitals to form new, equivalent orbitals (sp3, sp2, sp).
• Bond order — Number of bonding pairs between atoms; higher means shorter, stronger bond.

Action Items / Next Steps

• Review Lewis structures and VSEPR geometries in your lab notes.
• Study key examples: NH3, BF3, NO2, CO3^2-, XeF4, benzene.
• Practice identifying hybridization and resonance in molecules.