Chemical Bonding and Molecular Structure
What Domain III Tests
This domain tests how atoms combine: ionic and covalent bonding, Lewis structures, formal charge, VSEPR geometry, hybridization, polarity, and intermolecular forces. Questions often ask you to translate a formula into a shape, bond angle, and set of forces, then predict a physical property such as boiling point or solubility.
Core Rules and Concepts
- Ionic bonds form between metals and nonmetals by electron transfer; covalent bonds form between nonmetals by sharing. Bond polarity depends on electronegativity difference; molecular polarity depends on both bond dipoles and geometry.
- Lewis structures: count valence electrons, connect atoms, distribute lone pairs, form multiple bonds if needed, and check formal charge. Formal charge = valence electrons − nonbonding electrons − ½ bonding electrons.
- VSEPR: electron domains repel and arrange to minimize repulsion. Two domains = linear (180°); three = trigonal planar (120°); four = tetrahedral (109.5°). Lone pairs occupy space and compress bond angles.
- Hybridization: steric number 2 = sp, 3 = sp², 4 = sp³. Sigma bonds are end-to-end; pi bonds are side-to-side and restrict rotation.
- Intermolecular forces in increasing strength: London dispersion < dipole–dipole < hydrogen bonding < ion–dipole. Stronger forces generally raise boiling point.
Worked Example
Determine the molecular geometry, approximate bond angle, and polarity of NH₃. Reasoning: N has 5 valence electrons; three H atoms contribute 3, for 8 total. The Lewis structure has three N–H single bonds and one lone pair on N. That gives four electron domains, so the electron geometry is tetrahedral. One domain is a lone pair, so the molecular geometry is trigonal pyramidal, with bond angles slightly less than 109.5° (about 107°). The N–H bonds are polar and the lone pair makes the molecule asymmetric, so the bond dipoles do not cancel: NH₃ is polar and can hydrogen-bond.
Traps That Cost Marks
- Reporting electron geometry when the question asks for molecular geometry.
- Assuming a molecule with polar bonds must be polar; CO₂ has polar C=O bonds but is linear and nonpolar.
- Forgetting expanded octets are possible for period 3 and heavier elements, but not for N, O, or F.
- Ignoring formal charge when choosing among resonance structures.
- Ranking intermolecular forces by molecular size alone; hydrogen bonding requires H bonded to N, O, or F.