Unit 2 · Molecular and Ionic Compound Structure and Properties

Nonpolar covalent bond ● (core concept) — A bond in which electrons are shared nearly equally between atoms of similar electronegativity.

Polar covalent bond ● (core concept) — A bond in which electrons are shared unequally because the atoms have different electronegativities; partial charges (δ+ and δ−) appear, and the bond dipole grows with the electronegativity difference.

Bond dipole (partial charges) ● (core concept) — The separation of charge in a polar bond, written with δ+ on the less electronegative atom and δ− on the more electronegative atom.

Ionic bond ● (core concept) — A bond typically between a metal and a nonmetal in which one or more electrons are effectively transferred. Bonding is a continuum: even polar covalent bonds have some ionic character, and all ionic bonds retain some covalent character.

Metallic bonding ● (core concept) — Bonding in metals in which valence electrons are delocalized over an array of positive metal ions rather than being associated with individual atoms.

Potential energy vs. distance graph ● (core concept) — A plot of a molecule's potential energy against the distance between two atoms; it shows a minimum at the equilibrium bond length and the depth of the well equals the bond energy.

Equilibrium bond length ● (core concept) — The separation between two bonded atoms at which the molecule's potential energy is at its minimum.

Bond energy ● (core concept) — The energy required to separate two bonded atoms; stronger bonds have deeper potential-energy wells.

Bond order ● (core concept) — The number of shared electron pairs between two atoms (single = 1, double = 2, triple = 3); higher bond order means shorter, stronger bonds.

Coulomb's law and ionic bond strength ● (core concept) — The attraction between ions is proportional to the product of their charges and inversely proportional to the square of the distance between them, so larger charges and smaller ions make stronger ionic bonds.

Ionic crystal lattice ● (core concept) — A three-dimensional array of ions arranged to maximize attraction between opposite charges and minimize repulsion between like charges, as predicted by Coulomb's law.

Sea of electrons model ● (core concept) — A model of metallic bonding in which an ordered array of positive metal ions is surrounded by delocalized valence electrons.

Interstitial alloy ● (core concept) — An alloy formed when smaller atoms occupy the spaces between larger metal atoms; the distorted lattice is more rigid. Example: carbon in iron makes steel.

Substitutional alloy ● (core concept) — An alloy formed when atoms of a similar size replace some of the metal atoms in the lattice. Example: zinc replacing copper makes brass.

Lewis diagram (Lewis structure) ● (core concept) — A representation of a molecule or ion showing valence electrons and the bonds between atoms, constructed by a set of established rules.

Resonance ● (core concept) — A description used when more than one equivalent Lewis structure can be drawn for a molecule or ion; the real structure is an average of the resonance forms.

Formal charge ● (core concept) — A bookkeeping value for each atom in a Lewis structure, used to choose among nonequivalent Lewis structures (the best structure minimizes formal charges).

Octet rule ● (core concept) — The guideline that atoms in a Lewis structure ideally have eight valence electrons; it has exceptions, such as species with an odd number of valence electrons.

VSEPR theory ● (core concept) — Valence-shell electron-pair repulsion theory: electron domains around a central atom arrange themselves to minimize repulsion, which predicts molecular geometry.

Molecular geometries ● (core concept) — The standard shapes predicted by VSEPR: linear, trigonal planar, tetrahedral, trigonal pyramidal, bent, trigonal bipyramidal, seesaw, T-shaped, octahedral, square pyramidal, and square planar.

Bond angle ● (core concept) — The angle between two bonds to a central atom, determined by the molecular geometry.

Molecular dipole moment ● (core concept) — A molecule is polar only if it has polar bonds AND their dipoles do not cancel due to symmetry; molecular geometry therefore determines polarity.

Hybridization (hybrid orbitals) ● (core concept) — The mixing of atomic orbitals to form sp (180°), sp² (120°), or sp³ (109.5°) hybrid orbitals, matching the observed geometry around an atom.

Sigma (σ) bond ● (core concept) — A bond formed by head-on orbital overlap; it has greater bond energy than a pi bond and is the only bond type in single bonds.

Pi (π) bond ● (core concept) — A bond formed by sideways orbital overlap; it is weaker than a sigma bond and prevents rotation, which can produce geometric isomers.

Geometric isomers ● (core concept) — Molecules with the same formula but different spatial arrangements, made possible when pi bonds prevent rotation about a bond.