Chemical Bonding Study Guide
Bonding connects atomic structure to the shapes and properties of real molecules. The workflow is always the same: identify the bond type, draw the Lewis structure, check formal charges, consider resonance, predict the geometry with VSEPR, and judge polarity from electronegativity and shape. Each step has a classic error — expanded octets on period-2 atoms, unminimized formal charges, geometry confusion, polarity-by-assumption. This guide walks the whole chain and flags every trap.
Key formulas
Formal charge = valence electrons − (nonbonding electrons + 1/2 bonding electrons)
Octet rule: atoms tend to have 8 valence electrons; H aims for 2; period 3+ elements may expand the octet
VSEPR shapes: 2 domains = linear; 3 = trigonal planar; 4 = tetrahedral; 5 = trigonal bipyramidal; 6 = octahedral
Bond polarity guide: ΔEN < 0.4 nonpolar covalent; 0.4–1.7 polar covalent; > 1.7 often ionic
Molecular polarity: net dipole exists only when polar bonds are arranged asymmetrically
Resonance: real structure is a hybrid of contributing forms differing only in electron placement
Topic checklist
Bond types
Ionic bonding: electron transfer from metal to nonmetal, forming a lattice of oppositely charged ions.
Covalent bonding: electron sharing between nonmetals, forming discrete molecules.
Metallic bonding: a sea of delocalized electrons around positive ions, explaining conductivity and malleability.
Lewis structures
Draw the skeleton, count total valence electrons, add bonds and lone pairs, then check the octet.
Hydrogen gets 2 electrons; most other main-group atoms aim for 8.
Multiple bonds are used when single bonds leave atoms short of the octet.
Formal charge
Formal charge = valence electrons − (nonbonding electrons + 1/2 bonding electrons).
The best structure minimizes formal charges and puts negative charges on the more electronegative atom.
Adjacent like charges and large separations of charge are red flags.
Resonance
Resonance: when multiple valid Lewis structures differ only in electron placement, the real molecule is a hybrid of them.
Curved arrows show electron movement between resonance forms.
Resonance stabilizes molecules and explains equal bond lengths, e.g., in ozone and benzene.
VSEPR and geometry
VSEPR: electron domains (bonds + lone pairs) repel and arrange to maximize separation.
Lone pairs compress bond angles: CH4 (109.5°), NH3 (~107°), H2O (~104.5°).
Electron geometry counts lone pairs; molecular geometry describes only atom positions.
Bond and molecular polarity
Bond polarity comes from electronegativity difference: ΔEN < 0.4 nonpolar, 0.4–1.7 polar covalent, > 1.7 often ionic.
A molecule is polar only if it has polar bonds AND an asymmetric shape that leaves a net dipole.
Symmetry cancels dipoles: CO2 and CCl4 are nonpolar despite polar bonds.
Common mistakes
Mistake: Giving period-2 elements more than 8 electrons (expanded octet)
Fix:
The octet rule is strict for period-2 elements (C, N, O, F): never give them more than 8 electrons. Expanded octets are allowed only from period 3 onward (P, S, Cl, ...).
Mistake: Accepting the first Lewis structure without minimizing formal charge
Fix:
Calculate formal charges for every candidate structure and choose the one with charges closest to zero, negative charge on the most electronegative atom. Don't stop at the first structure that works.
Mistake: Confusing electron geometry with molecular geometry
Fix:
Electron geometry includes lone pairs (e.g., tetrahedral electron geometry for NH3); molecular geometry names only the atoms (trigonal pyramidal). State both when asked.
Mistake: Calling a molecule polar just because it contains polar bonds
Fix:
Check two things: are the bonds polar, and does the shape cancel their dipoles? A symmetric molecule like CO2 has polar bonds but zero net dipole.
Exam tips
Drill bonding-practice for Lewis structures and vsepr-practice for shape naming — both are high-volume exam topics.
For every Lewis structure, write the formal charges before deciding it is final; examiners reward the best structure.
Memorize the five core geometries: linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral.
On polarity questions, draw the shape first, then add dipole arrows — the answer follows from the picture.
Pair with periodic-trends-practice: electronegativity fluency makes bond-polarity calls instant.