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Credit Sprint's Guide to Chemistry Unit 2: Molecular and Ionic Bonding

AP® weight 7–9%CLEP® weight 8–12%

Atoms bond to reach lower energy. Ionic bonding is electrostatic attraction in a lattice, covalent bonding shares electron pairs, and metallic bonding shares a sea of electrons. Lewis structures and VSEPR predict shape and polarity, and shape explains properties.

Electron domain geometriesA Linear180°CO₂A Trigonal planar120°BF₃A Tetrahedral109.5°CH₄A Bent104.5°H₂O

What you need to know

  • Ionic compounds form lattices; lattice energy rises with ion charge and falls with ion size (Coulomb's law). MgO melts far higher than NaCl.
  • Ionic solids are brittle and conduct only when molten or dissolved. Metals conduct, and are malleable because the electron sea holds shifting layers together.
  • Alloys: interstitial (small atoms in gaps, like carbon in steel) and substitutional (similar-size atoms swap, like zinc in brass).
  • Draw Lewis structures: count valence electrons, make bonds, complete octets, use formal charge to choose the best structure. Resonance spreads bonding over several structures.
  • Exceptions: BF₃ (incomplete octet), PCl₅ and SF₆ (expanded octet), NO (odd electron).
  • VSEPR: electron domains repel. Two domains: linear 180°. Three: trigonal planar 120°. Four: tetrahedral 109.5° (bent or pyramidal with lone pairs, smaller angles). Five: trigonal bipyramidal. Six: octahedral.
  • Hybridization follows domains: sp (2), sp² (3), sp³ (4). A double bond is one σ and one π; a triple bond is one σ and two π.
  • Bond order up means shorter and stronger bonds: C≡C < C=C < C–C in length.
  • A molecule is polar only if bond dipoles do not cancel. CO₂ and CCl₄ are nonpolar; H₂O and NH₃ are polar.
  • Network covalent solids (diamond, SiO₂) have very high melting points; graphite conducts along its layers.

Equations and rules

formal charge = valence e⁻ − nonbonding e⁻ − ½(bonding e⁻)
bond order = number of bonds between two atoms

Exam traps

Watch forPolar bonds do not make a polar molecule if the shape is symmetric.
Watch forElectron geometry counts lone pairs; molecular geometry names only the atoms.
Watch forIonic compounds don't conduct as solids because ions can't move.
Watch forResonance structures are not flipping back and forth; the real bonding is an average.

Key terms

  • Ionic bond: Electrostatic attraction between cations and anions in a lattice.
  • Lattice energy: Energy released when gaseous ions form a solid; larger with higher charges and smaller ions.
  • Covalent bond: A shared pair of electrons between atoms.
  • Metallic bond: Attraction between metal cations and a sea of delocalized electrons.
  • Lewis structure: A diagram of bonds and lone pairs showing valence electrons.
  • Formal charge: Valence electrons − nonbonding electrons − half the bonding electrons.
  • Resonance: Two or more valid Lewis structures for one species; the real structure is a hybrid.
  • VSEPR: Electron domains around a central atom repel and arrange to be as far apart as possible.
  • Bond angle for tetrahedral electron geometry: 109.5°; lone pairs squeeze the bond angles smaller.
  • Hybridization: Mixing of atomic orbitals: sp (linear), sp² (trigonal planar), sp³ (tetrahedral).
  • Sigma and pi bonds: Sigma: head-on overlap, one per bond. Pi: sideways p overlap, found in double and triple bonds.
  • Polar molecule: A molecule whose bond dipoles do not cancel, giving a net dipole.
  • Interstitial alloy: Small atoms fill gaps in a metal lattice, as carbon does in steel.
  • Network covalent solid: Atoms joined in a continuous covalent lattice: very hard and high melting, like diamond.

Practice questions

Five of the 52 Unit 2 questions in Credit Sprint. Every choice is explained in the app.

1. Which compound should have the highest melting point?
  1. NaCl
  2. KBr
  3. LiF
  4. MgO
Show answer

MgO. Highest ion charges (2+/2−) and small ions give the strongest Coulombic attraction.

2. What is the formal charge on nitrogen in NH₄⁺?
  1. 0
  2. +1
  3. −1
  4. +4
Show answer

+1. Nitrogen has 5 valence electrons and owns 4 bonds: 5 − 4 = +1.

3. What is the molecular geometry of ClF₃?
  1. See-saw
  2. Trigonal pyramidal
  3. Trigonal planar
  4. T-shaped
Show answer

T-shaped. Five domains (trigonal bipyramidal) with two lone pairs in equatorial positions.

4. What is the hybridization of carbon in methane?
  1. sp³
  2. sp²
  3. sp³d²
  4. sp
Show answer

sp³. Four bonding domains.

5. Which molecule is polar?
  1. NH₃
  2. CO₂
  3. CCl₄
  4. BF₃
Show answer

NH₃. The pyramidal shape means the bond dipoles don't cancel.