Chemical Bonding and Molecular Structure
What this domain tests
Chemical Bonding and Molecular Structure tests how atoms combine and how the resulting geometry determines properties: ionic versus covalent bonding, Lewis structures, VSEPR shapes, bond and molecular polarity, and intermolecular forces.
Bond types
- Ionic bonds form by electron transfer, usually between a metal and a nonmetal; the lattice is held together by electrostatic attraction, giving high melting points and conductivity when molten or dissolved.
- Covalent bonds share electrons between nonmetals. Electronegativity difference is a guideline: roughly below 0.4 nonpolar covalent, 0.4 to 1.7 polar covalent, and above 1.7 ionic.
- Metallic bonding is a lattice of cations in a delocalized electron sea, which explains conductivity and malleability.
Lewis structures and VSEPR
- Count total valence electrons; hydrogen has a duet and period-2 elements an octet. Formal charge = valence electrons − nonbonding electrons − ½ bonding electrons.
- Electron domains (bonds and lone pairs; double and triple bonds count as one domain) set electron geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral.
- Lone pairs repel more strongly than bonding pairs, shrinking bond angles: CH4 109.5°, NH3 107°, H2O 104.5°.
- Resonance delocalizes electrons; the real structure is a hybrid with fractional bond order.
Polarity and intermolecular forces
- A molecule is polar only if its bond dipoles do not cancel. CO2 and CCl4 contain polar bonds but are nonpolar overall because of symmetry.
- Intermolecular force strength: ion–dipole > hydrogen bonding > dipole–dipole > London dispersion. Hydrogen bonding requires H bonded directly to N, O, or F.
- Stronger intermolecular forces raise boiling points and lower vapor pressure; dispersion forces grow with electron count and surface area.
Worked example
Determine the molecular geometry and polarity of NH3. Nitrogen contributes 5 valence electrons and each hydrogen contributes 1, giving 8 electrons. The Lewis structure has three N–H bonds and one lone pair on nitrogen. Four electron domains give tetrahedral electron geometry; the lone pair makes the molecular geometry trigonal pyramidal with angles near 107°. Nitrogen (3.04) and hydrogen (2.20) differ by 0.84, so each N–H bond is polar; the lone pair prevents cancellation, so NH3 is polar and hydrogen bonds.
Traps that cost marks
- Reporting electron-domain geometry when molecular geometry is requested.
- Assuming polar bonds always produce a polar molecule.
- Forgetting lone pairs on the central atom.
- Claiming hydrogen bonding for HCl or H2S.
- Treating the metal/nonmetal rule as absolute rather than a guideline.