Course review

2.2 The covalent model

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Learning objective

2.2.1—Covalent bonding

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• Electrostatic attraction between shared electron pair and nuclei • Octet rule: tendency to achieve 8 valence electrons • Lewis formulas (up to 4 electron pairs per atom)

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Learning objective

2.2.2—Bond types and strength

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• Single, double, triple bonds (1, 2, 3 shared pairs) • More bonds → shorter length, stronger bond

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Learning objective

2.2.3—Coordination bonds

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• Both electrons from same atom • Identify coordination bonds; include transition element complexes at HL

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Learning objective

2.2.4—VSEPR model

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• Predict molecular shapes from electron domain repulsion • Electron domain and molecular geometry (up to 4 domains) • Include bond angles and lone-pair effects for common shapes

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Learning objective

2.2.5—Bond polarity

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• Results from electronegativity differences • Deduce polar bonds from electronegativity values and show bond dipoles

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Learning objective

2.2.6—Molecular polarity

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• Depends on bond polarity + molecular geometry • Net dipole moment • Identify when bond dipoles cancel or produce a polar molecule/ion

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Learning objective

2.2.7—Covalent network structures

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• Carbon allotropes: diamond, graphite, fullerenes, graphene • Silicon and silicon dioxide

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Learning objective

2.2.8—Intermolecular forces (IMF)

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• London dispersion forces • Dipole-induced dipole • Dipole-dipole • Hydrogen bonding • Deduce IMF types from molecular size and polarity

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Learning objective

2.2.9—IMF strength and properties

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• Relative strength: London < dipole-dipole < hydrogen bonding • Effects on volatility, conductivity, solubility • Explain properties of covalent substances using IMF strength and molar mass

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Learning objective

2.2.10—Chromatography

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• Separates components based on IMF attractions • Calculate and interpret RF values • Link mobile/stationary phase attraction to separation; operational details are not assessed

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Learning objective

2.2.11 (HL)—Resonance structures

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• Multiple possible positions for double bonds • Deduce resonance structures and use the term delocalization

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Learning objective

2.2.12 (HL)—Benzene (C₆H₆)

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• Important resonance example

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2.2.13 (HL)—Expanded octets

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• Atoms with 5 or 6 electron domains • VSEPR for these species • Draw Lewis formulas and geometries for five- and six-domain species

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2.2.14 (HL)—Formal charge

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• Determine preferred Lewis formula • Use formal charge to compare valid Lewis structures

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Learning objective

2.2.15 (HL)—Sigma (σ) and pi (π) bonds

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• σ bonds: head-on orbital combination, electron density along bond axis • π bonds: lateral p-orbital combination, electron density on opposite sides

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2.2.16 (HL)—Hybridization

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• Mixing atomic orbitals to form hybrid orbitals • Relationships: Lewis formulas → electron domains → geometry → hybridization • Analyse hybridization and bond formation in molecules and ions

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