Chemical Bonding and Molecular Structure
The Chemical Bonding and Molecular Structure domain tests how atoms combine, how Lewis structures and VSEPR predict shape, and how polarity and intermolecular forces explain physical properties. Expect items on ionic versus covalent bonding, formal charge, resonance, hybridization, bond order, and hydrogen bonding.
Core rules and concepts
- Ionic bonding involves electron transfer and electrostatic attraction; covalent bonding involves shared electron pairs.
- Lewis structures minimize formal charge and obey the octet rule except for H, Be, and expanded-octet period 3+ centers.
- VSEPR counts electron domains (bonds plus lone pairs); lone pairs repel more strongly and lower bond angles.
- Molecular polarity depends on both bond dipoles and geometry; symmetric cancellation gives nonpolar molecules.
- Intermolecular forces rank roughly as ion-dipole, hydrogen bonding, dipole-dipole, and London dispersion, though size and polarizability matter.
Worked example
Predict the shape and polarity of H₂O. Oxygen has six valence electrons; two bonding pairs to H and two lone pairs give four electron domains. The electron-domain geometry is tetrahedral, but the molecular geometry is bent. The O–H bond dipoles point toward oxygen and do not cancel because the bend is about 104.5°. Water is polar and forms hydrogen bonds, explaining its high boiling point relative to similar-sized nonpolar molecules.
Traps that cost marks
- Naming the electron-domain geometry instead of the molecular geometry.
- Assuming polar bonds always produce a polar molecule.
- Giving period 2 atoms expanded octets.
- Ignoring formal charge when choosing among resonance structures.
- Treating all intermolecular forces as equal in strength.