S2.2 The Covalent Model

• A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei.

• The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons.

• Single, double and triple bonds involve one, two and three shared pairs of electrons respectively.

• A coordination bond is a covalent bond in which both the electrons of the shared pair originate from the same atom.

• The valence shell electron pair repulsion (VSEPR) model enables the shapes of molecules to be predicted from the repulsion of electron domains around a central atom.

• Bond polarity results from the difference in electronegativities of the bonded atoms.

• Molecular polarity depends on both bond polarity and molecular geometry.

• Carbon and silicon form covalent network structures.

• The nature of the force that exists between molecules is determined by the size and polarity of the molecules. Intermolecular forces include London (dispersion), dipole-induced dipole, dipole–dipole and hydrogen bonding.

• Given comparable molar mass, the relative strengths of intermolecular forces are generally: London (dispersion) forces < dipole–dipole forces < hydrogen bonding.

• Chromatography is a technique used to separate the components of a mixture based on their relative attractions involving intermolecular forces to mobile and stationary phases.

Nature of science—What are some of the limitations of the octet rule?

Structure 1.3—Why do noble gases form covalent bonds less readily than other elements?

Structure 2.1—Why do ionic bonds only form between different elements while covalent bonds can form between atoms of the same element?

Reactivity 2.2—How does the presence of double and triple bonds in molecules influence their reactivity?

AHL Reactivity 3.4—Why do Lewis acid–base reactions lead to the formation of coordination bonds?

Nature of science—How useful is the VSEPR model at predicting molecular geometry?

Structure 2.1—What properties of ionic compounds might be expected in compounds with polar covalent bonding?

AHL Structure 3.2—What features of a molecule make it “infrared (IR) active”?

Structure 3.1—Why are silicon–silicon bonds generally weaker than carbon–carbon bonds?

Structure 1.5—To what extent can intermolecular forces explain the deviation of real gases from ideal behaviour?

Nature of science, Structure 1.1, 2.1, 2.3—How do the terms “bonds” and “forces” compare?

Nature of science—How can advances in technology lead to changes in scientific definitions,

e.g. the updated International Union of Pure and Applied Chemistry (IUPAC) definition of the hydrogen bond?

Tool 1, Inquiry 2—What experimental data demonstrate the physical properties of covalent substances?

Structure 3.2—To what extent does a functional group determine the nature of the intermolecular forces?

Tool 1—How can a mixture be separated using paper chromatography or thin layer chromatography (TLC)?