Structure 2.2 The covalent model 
Structure 2.2.1 and 2.2.2
​Understandings:
  • A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei (2.2.1). 
  • The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons (2.2.1). 
  • Single, double and triple bonds involve one, two and three shared pairs of electrons respectively (2.2.2). 
Learning outcomes:
  • Deduce the Lewis formula of molecules and ions for up to four electron pairs on each atom (2.2.1).
  • Explain the relationship between the number of bonds, bond length and bond strength (2.2.2). 
Additional notes:
  • Lewis formulas (also known as electron dot or Lewis structures) show all the valence electrons (bonding and non-bonding pairs) in a covalently bonded species.
  • Electron pairs in a Lewis formula can be shown as dots, crosses or dashes.
  • Molecules containing atoms with fewer than an octet of electrons should be covered.
  • Organic and inorganic examples should be used.​
Linking questions:
  • 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?
This video covers the octet rule and the exceptions to the octet rule. 
This video covers covalent bonding. 
This video covers Lewis structures. 
Structure 2.2.3
​Understandings:
  • A coordination bond is a covalent bond in which both the electrons of the shared pair originate from the same atom.
Learning outcomes:
  • Identify coordination bonds in compounds.
Additional notes:
  • Include coverage of transition element complexes (HL).​
Linking questions:
  • Reactivity 3.4 (HL) Why do Lewis acid–base reactions lead to the formation of coordination bonds?
This video covers coordinate covalent bonds (note that the IB now refers to these type of bonds as coordination bonds according to the IUPAC definition). 
Structure 2.2.4
​Understandings:
  • 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.
Learning outcomes:
  • Predict the electron domain geometry and the molecular geometry for species with up to four electron domains.
Additional notes:
  • Include predicting how non-bonding pairs and multiple bonds affect bond angles.​​ ​
This video covers using the VSEPR theory to predict the molecular and electron domain geometries for atoms and ions with up to four electron domains. 
Structure 2.2.5
​Understandings:
  • Bond polarity results from the difference in electronegativities of the bonded atoms.
Learning outcomes:
  • Deduce the polar nature of a covalent bond from electronegativity values.
Additional notes:
  • Bond dipoles can be shown either with partial charges or vectors.
  • Electronegativity values are given in the data booklet.​
Linking questions:
  • Structure 2.1 What properties of ionic compounds might be expected in compounds with polar covalent bonding?
This video covers polar and non-polar covalent bonds. 
Structure 2.2.6
​Understandings:
  • Molecular polarity depends on both bond polarity and molecular geometry.
Learning outcomes:
  • Deduce the net dipole of a molecule or ion by considering bond polarity and geometry.
Additional notes:
  • Examples should include species in which bond dipoles do and do not cancel each other.​
Linking questions:
  • Structure 3.2 (HL) What features of a molecule make it “infrared (IR) active”?
This video covers polar and non-polar molecules.
Structure 2.2.7
​Understandings:
  • Carbon and silicon form covalent network structures.
Learning outcomes:
  • Describe the structures and explanation of the properties of silicon, silicon dioxide and carbon’s allotropes: diamond, graphite, fullerenes and graphene.
Additional notes:
  • Allotropes of the same element have different bonding and structural patterns, and so have different chemical and physical properties.​
Linking questions:
  • Structure 3.1 Why are silicon–silicon bonds generally weaker than carbon–carbon bonds?
This video covers the properties of simple molecular substances and giant covalent structures (note that the IB now refers to giant covalent structures as covalent network structures).
This video covers the allotropes of carbon.
Structure 2.2.8 and 2.2.9
​Understandings:
  • 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.
Learning outcomes:
  • Deduce the types of intermolecular force present from the structural features of covalent molecules.
  • Explain the physical properties of covalent substances to include volatility, electrical conductivity and solubility in terms of their structure.
Additional notes:
  • The term “van der Waals forces” should be used as an inclusive term to include dipole–dipole, dipole- induced dipole, and London (dispersion) forces.
  • Hydrogen bonds occur when hydrogen, being covalently bonded to an electronegative atom, has an attractive interaction on a neighbouring electronegative atom.​
Linking questions:
  • Structure 1.5 To what extent can intermolecular forces explain the deviation of real gases from ideal behaviour?
  • ​Structure 3.2 To what extent does a functional group determine the nature of the intermolecular forces?
This video covers intermolecular forces.
This video covers solubility and intermolecular forces.
This video covers electrical conductivity (note that metallic bonding is covered in structure 2.3).
Structure 2.2.10
​Understandings:
  • 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.
Learning outcomes:
  • Explain, calculate and interpret the retardation factor values, RF.​
Additional notes:
  • The use of locating agents is not required.
  • The operational details of a gas chromatograph or high-performance liquid chromatograph will not be assessed.
Video coming soon