Flashcards · IB Diploma Programme Chemistry · Topic 2

Structure 2 – Models of bonding and structure flashcards

31 cards on Topic 2, subtopics 2.1 to 2.4. Pick a subtopic or study them all.

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2.1 The ionic model

Ionic bond: Electrostatic attraction between oppositely charged ions.
Structure of an ionic compound: Giant 3D lattice; each ion surrounded by oppositely charged ions; attractions act in all directions.
Why is the formula of an ionic compound an empirical formula?: Because a crystal has no fixed number of ions and no discrete molecules.
Formulas of rubidium nitride and iron(III) carbonate: Rb⁺ and N³⁻: Rb₃N. Fe³⁺ and CO₃²⁻: Fe₂(CO₃)₃.
Why does CaS have a larger lattice enthalpy than K₂S?: Both contain S²⁻, but Ca²⁺ has a higher charge and is smaller than K⁺, so the ion centres are closer and the electrostatic attraction is stronger. Charge usually dominates.
Electrical conductivity: molten vs aqueous vs solid ionic compound: The first two conduct, the last does not. Ions move; electrons do not.
The seven polyatomic ions to know: Ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, hydrogencarbonate HCO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, phosphate PO₄³⁻.

2.2 The covalent model

Covalent bond, and coordination bond: Covalent: two nuclei both attracting the same shared electron pair. Coordination: a covalent bond where both shared electrons come from the same atom (H₃O⁺, NH₄⁺, Al₂Cl₆, CO). Once formed it is identical to the others.
Shapes and approximate angles of BF₃, NH₃ and OF₂: BF₃: 3 domains, 0 lone pairs, trigonal planar, 120°. NH₃: 4 domains, 1 lone pair, trigonal pyramidal, about 107°. OF₂: 4 domains, 2 lone pairs, bent, about 104.5°.
Single, double and triple bonds between the same two atoms: length and strength: More shared pairs means more electron density between the nuclei: triple is shortest and strongest, single is longest and weakest.
Why does SiF₄ have polar bonds but no net dipole?: It is tetrahedral with identical outer atoms, so the bond dipoles cancel. Lone pairs on the central atom (pyramidal, bent) or different outer atoms usually prevent cancelling.
Intermolecular forces in order of strength (similar molar mass): London (always present) < dipole-induced dipole < dipole–dipole < hydrogen bond (H on N, O or F plus a lone pair on N, O or F).
Diamond, graphite, graphene and C₆₀: bonds per carbon and conductivity: Diamond: 4, does not conduct. Graphite: 3 in layers, conducts along the layers. Graphene: 3 in one sheet, conducts very well. C₆₀: 3 in a cage, molecular, conducts poorly.
Resonance: Two or more valid Lewis formulas differ only in where π electrons sit. The real species is one resonance hybrid; its delocalized bonds are equal in length, between single and double (e.g. 1.5 for three pairs over two positions). (HL only)
Formal charge: formula and how to choose the preferred Lewis formula: FC = V − (L + ½B). Prefer formal charges closest to zero; if tied, put the negative formal charge on the more electronegative atom. (HL only)
σ vs π bonds: Head-on vs sideways; on the axis vs either side of it. A single bond is 1 σ; a double bond 1 σ + 1 π; a triple bond 1 σ + 2 π. (HL only)
Hybridization for 4, 3 and 2 electron domains: 4 domains: sp³, tetrahedral, 109.5°. 3: sp², trigonal planar, 120° (one p left for a π bond). 2: sp, linear, 180° (two p left). Count domains, not atoms. (HL only)
Molecular geometry of XeF₄ and ClF₃: XeF₄: 6 domains, 2 lone pairs opposite each other, square planar. ClF₃: 5 domains, 2 lone pairs equatorial, T-shaped. (HL only)

2.3 The metallic model

Metallic bond: The electrostatic attraction holding a lattice of cations to the delocalized electrons around them. The bonding is non-directional.
How the metallic model explains electrical and thermal conductivity: Electrical: under a potential difference the delocalized electrons drift, carrying charge; the cations stay put. Thermal: mobile electrons gain kinetic energy at the hot end and carry it to cooler regions.
Why are metals malleable and ductile?: Layers of cations slide over each other and the delocalized electrons move with them, so the attraction is kept and the metal reshapes instead of breaking.
What makes a metallic bond stronger?: A higher cation charge (more delocalized electrons per atom) and a smaller ion radius. Example: melting point rises Na → Mg → Al.
Why does strontium melt higher than rubidium?: Sr gives Sr²⁺ and two delocalized electrons per atom; Rb gives Rb⁺ and one. Sr²⁺ is also smaller, so the attraction is stronger and more energy is needed (about 1050 K vs about 312 K).
Why do transition elements have high melting points and conduct well?: 3d as well as 4s electrons can be delocalized, giving greater electron density and higher cation charge with small ions: very strong metallic bonding and more mobile charge carriers. (HL only)

2.4 From models to materials

The bonding triangle: what the axes show: Vertical: electronegativity difference, Δχ. Horizontal: average electronegativity, (χA + χB) ÷ 2. Top: ionic; bottom left: metallic; bottom right: covalent.
Place NaBr on the bonding triangle (χ: Na 0.93, Br 2.96): Δχ = 2.03; average = 1.945. Ionic region: high melting point, conducts when molten.
Why is an alloy harder than the pure metal?: Atoms of a different size distort the layers of the lattice, so the layers resist sliding. The delocalized electrons remain, so the alloy still conducts.
Why are most plastics insulators that soften on heating?: No mobile ions or delocalized electrons, so no conduction. Heating overcomes only the intermolecular forces between chains, so they are mouldable (cross-linked ones do not soften).
Repeating unit of the addition polymer from 3-chloropropene, CH₂=CH–CH₂Cl: –[–CH₂–CH(CH₂Cl)–]–ₙ. The C=C becomes C–C; CH₂Cl is a side chain. Addition polymerization has 100% atom economy.
Polyesters vs polyamides: Polyester: –COOH + –OH, ester link –COO–. Polyamide: –COOH + –NH₂, amide link –CONH–. Both are condensation polymers that release water at each link. (HL only)
How many water molecules are released when N monomer units form a condensation polymer chain?: N − 1 links, so N − 1 water molecules; the same number is needed to hydrolyse it completely. Proteins, polysaccharides and nucleic acids all form this way. (HL only)