AP Chemistry Unit 2 Notes: Building Molecules from Lewis Structures to 3D Geometry

Lewis diagram (Lewis structure)

A 2D electron-bookkeeping drawing that shows valence electrons in a molecule/ion as bond lines (bonding pairs) and lone-pair dots (nonbonding pairs), sometimes including formal charges.

Valence electrons

The outer-shell electrons of an atom that are counted and distributed when drawing Lewis structures and determining bonding.

Octet guideline

The guideline that many main-group atoms (especially C, N, O, F) tend to be most stable with 8 electrons around them when counting bonding and lone-pair electrons.

Duet rule

Hydrogen’s exception to the octet guideline; H is stable with 2 electrons (one bonding pair).

Skeletal structure

The initial connectivity framework in a Lewis structure showing which atoms are bonded to which (usually with the least electronegative atom, not H, as central).

Terminal atom

An atom placed on the outside of a skeletal structure; it is typically given its octet before adding electrons to the central atom.

Valence electron counting with ionic charge

The method of adjusting total valence electrons by adding 1 electron per negative charge or subtracting 1 electron per positive charge.

Multiple bond formation (Lewis structures)

Creating double or triple bonds by converting a lone pair on an adjacent atom into a bonding pair when the central atom lacks an octet (without changing the total electron count).

Incomplete octet (electron-deficient center)

A stable Lewis arrangement where a central atom has fewer than 8 electrons (e.g., Be with 4 electrons; B/Al with 6 electrons) and is not “forced” to an octet by inventing electrons.

Odd-electron species (radical)

A molecule/ion with an odd number of total valence electrons, requiring at least one unpaired electron and preventing all atoms from having perfect octets.

Expanded octet

A common Lewis depiction for Period 3 or lower atoms (e.g., P, S, Cl, Br, I, Xe) showing more than 8 electrons around the central atom in some stable structures (e.g., SF6, PCl5).

Resonance

The use of multiple valid Lewis structures (resonance forms) to represent one real species when electrons are delocalized over multiple atoms.

Resonance hybrid

The real, averaged electron distribution implied by resonance forms; it is not a rapid switching between different molecules.

Equivalent resonance forms

Resonance structures with the same energy/validity (often created by moving a double bond among identical atoms), implying equal bond lengths in the actual molecule.

Resonance rule: what can change

In resonance, only electrons (lone pairs, π bonds, formal charges) move while the atom positions/skeletal connectivity stay the same.

Formal charge (FC)

A bookkeeping charge assigned to an atom in a Lewis structure assuming equal sharing of bonding electrons; used to compare competing structures.

Formal charge formula

FC = V − (N + B/2), where V is valence electrons (neutral atom), N is nonbonding electrons, and B is bonding electrons around that atom.

Best-Lewis-structure criteria (formal charge)

Preferred structures minimize the magnitude of formal charges, minimize charge separation, and place negative charge on more electronegative atoms when possible.

VSEPR theory

Valence Shell Electron Pair Repulsion theory: electron domains around a central atom arrange to minimize repulsions, determining 3D geometry.

Electron domain

A region of electron density around a central atom; each single, double, or triple bond counts as 1 domain, and each lone pair counts as 1 domain.

Electron geometry

The arrangement of all electron domains (bonding + lone pairs) around a central atom (e.g., linear, trigonal planar, tetrahedral).

Molecular geometry

The arrangement of atoms only (ignore lone pairs when naming the shape), though lone pairs still affect bond angles by repulsion.

Axial vs. equatorial (trigonal bipyramidal)

In trigonal bipyramidal electron geometry, equatorial positions (3) are 120° apart; axial positions (2) are 180° from each other and 90° to equatorial; lone pairs prefer equatorial positions.

Hybridization (AP model)

A bonding model where central-atom orbitals mix to match electron-domain geometry (2→sp, 3→sp2, 4→sp3, 5→sp3d, 6→sp3d2), describing the σ-bond framework.

Sigma bond vs. pi bond

A sigma (σ) bond is end-to-end overlap along the internuclear axis; a pi (π) bond is side-to-side overlap of unhybridized p orbitals; double bonds are 1σ+1π and triple bonds are 1σ+2π.