Chemical Bonding and Molecular Structure
What to know
This domain covers ionic, covalent, and metallic bonding; Lewis structures; VSEPR geometry; polarity; and intermolecular forces (London dispersion, dipole-dipole, hydrogen bonding). You must predict shapes and bond angles from electron domains and connect structure to properties such as boiling point and solubility.
Know that ionic compounds form between metals and nonmetals with large electronegativity differences, that covalent bonds share electrons, and that metallic bonding explains conductivity and malleability. Be able to draw Lewis structures including resonance and expanded octets, and count electron domains (bonding + lone pairs) to assign geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, with lone pairs compressing angles.
Traps
- Counting lone pairs as domains but forgetting they change shape, not just angle (e.g., water is bent, not linear).
- Assuming a molecule with polar bonds is polar overall; symmetry can cancel dipoles (CO2 is nonpolar).
- Ranking intermolecular forces incorrectly: hydrogen bonding requires H bonded to N, O, or F.
- Confusing intramolecular bonds with intermolecular forces when explaining boiling points.
- Overlooking formal charge when choosing among resonance structures.
Worked example
Predict the shape of NH3. Nitrogen has five valence electrons; three bonds to H plus one lone pair gives four electron domains, so the electron geometry is tetrahedral and the molecular shape is trigonal pyramidal with angles slightly less than 109.5°. The lone pair makes the molecule polar and enables hydrogen bonding, raising its boiling point relative to nonpolar molecules of similar mass.