Notes
Types of bonds
- Ionic: a metal transfers electrons to a nonmetal; ions held in a lattice. Lattice strength grows with larger charges and smaller ions (Coulomb's law). MgO has a stronger lattice than NaCl.
- Covalent: nonmetals share electrons. Polar if electronegativities differ; nonpolar if they're the same.
- Metallic: cations in a sea of delocalized electrons, which explains conductivity and malleability. Alloys are interstitial (small atoms fill gaps, like carbon in steel) or substitutional (similar-size atoms swap in, like brass).
- Potential energy curves: the bond length is at the energy minimum; the depth is the bond energy.
Lewis structures
1. Count total valence electrons (add for negative charge, subtract for positive). 2. Connect atoms with single bonds; the central atom is usually the least electronegative. 3. Complete octets on outer atoms, then the central atom; form double or triple bonds if needed.
- Formal charge $=$ valence electrons $-$ nonbonding electrons $\displaystyle - \frac{1}{2}$ bonding electrons. The best structure has formal charges closest to zero.
- Resonance: more than one valid structure; the real molecule is an average (like $\mathrm{O_3}$ or $\mathrm{NO_3^-}$).
- Exceptions: B can have 6 electrons; elements in period 3 and below can have expanded octets.
VSEPR shapes
| Electron domains | Lone pairs | Shape | Angle | Hybridization |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | sp |
| 3 | 0 | Trigonal planar | 120° | sp² |
| 4 | 0 | Tetrahedral | 109.5° | sp³ |
| 4 | 1 | Trigonal pyramidal | ~107° | sp³ |
| 4 | 2 | Bent | ~104.5° | sp³ |
| 5 | 0 | Trigonal bipyramidal | 90°, 120° | |
| 6 | 0 | Octahedral | 90° |
- A molecule is polar if it has polar bonds that don't cancel by symmetry ($\mathrm{H_2O}$ is polar; $\mathrm{CO_2}$ is nonpolar).
- A single bond is one sigma bond; a double bond is one sigma and one pi bond.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Mg²⁺ and O²⁻ have larger charges and are small.
Three bonding pairs and one lone pair.
Its two polar C=O bonds point in opposite directions and cancel.
Four electron domains around carbon.
A triple bond is one sigma bond and two pi bonds.
$5 + 3(6) + 1 = 24$.
Zinc and copper atoms are similar in size, so zinc substitutes into the lattice.
Atoms are most stable at the distance of lowest energy.
Free response
Write your answer first, then open the scoring guide.
Free-response question
Consider the molecule $\mathrm{SO_2}$.
(a) Draw a Lewis structure with formal charges of zero on all atoms. (Sulfur can expand its octet.)
Show the answer and scoring guide
O=S=O with two lone pairs on each oxygen and one lone pair on sulfur. 18 valence electrons; all formal charges are 0.
Check off the points you earned:
(b) Predict the molecular geometry and approximate O–S–O bond angle.
Show the answer and scoring guide
Three electron domains on S (two bonds and a lone pair): bent, slightly less than 120°.
Check off the points you earned:
(c) Is $\mathrm{SO_2}$ polar? Justify.
Show the answer and scoring guide
Yes. The S–O bonds are polar and the bent shape means the dipoles don't cancel.
Check off the points you earned: