2.2 The covalent model

Objective notes

16 learning objectives

2.2.1Covalent bonding

• 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)

2.2.2Bond types and strength

• Single, double, triple bonds (1, 2, 3 shared pairs)

• More bonds → shorter length, stronger bond

2.2.3Coordination bonds

• Both electrons from same atom

• Identify coordination bonds; include transition element complexes at HL

2.2.4VSEPR model

• 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

2.2.5Bond polarity

• Results from electronegativity differences

• Deduce polar bonds from electronegativity values and show bond dipoles

2.2.6Molecular polarity

• Depends on bond polarity + molecular geometry

• Net dipole moment

• Identify when bond dipoles cancel or produce a polar molecule/ion

2.2.7Covalent network structures

• Carbon allotropes: diamond, graphite, fullerenes, graphene

• Silicon and silicon dioxide

2.2.8Intermolecular forces (IMF)

• London dispersion forces

• Dipole-induced dipole

• Dipole-dipole

• Hydrogen bonding

• Deduce IMF types from molecular size and polarity

2.2.9IMF strength and properties

• Relative strength: London < dipole-dipole < hydrogen bonding

• Effects on volatility, conductivity, solubility

• Explain properties of covalent substances using IMF strength and molar mass

2.2.10Chromatography

• Separates components based on IMF attractions

• Calculate and interpret RF values

• Link mobile/stationary phase attraction to separation; operational details are not assessed

2.2.11(HL)—Resonance structures

• Multiple possible positions for double bonds

• Deduce resonance structures and use the term delocalization

2.2.12(HL)—Benzene (C₆H₆)

• Important resonance example

2.2.13(HL)—Expanded octets

• Atoms with 5 or 6 electron domains

• VSEPR for these species

• Draw Lewis formulas and geometries for five- and six-domain species

2.2.14(HL)—Formal charge

• Determine preferred Lewis formula

• Use formal charge to compare valid Lewis structures

2.2.15(HL)—Sigma (σ) and pi (π) bonds

• σ bonds: head-on orbital combination, electron density along bond axis

• π bonds: lateral p-orbital combination, electron density on opposite sides

2.2.16(HL)—Hybridization

• Mixing atomic orbitals to form hybrid orbitals

• Relationships: Lewis formulas → electron domains → geometry → hybridization

• Analyse hybridization and bond formation in molecules and ions