Covalent structures

Nature of science:

Scientists use models as representations of the real world—the development of the model of molecular shape (VSEPR) to explain observable properties. (1.10)

• Lewis (electron dot) structures show all the valence electrons in a covalently bonded species.
• The “octet rule” refers to the tendency of atoms to gain a valence shell with a total of 8 electrons.
• Some atoms, like Be and B, might form stable compounds with incomplete octets of electrons.
• Resonance structures occur when there is more than one possible position for a double bond in a molecule.
• Shapes of species are determined by the repulsion of electron pairs according to VSEPR theory.
• Carbon and silicon form giant covalent/network covalent structures.

Applications and skills:

• Deduction of Lewis (electron dot) structure of molecules and ions showing all valence electrons for up to four electron pairs on each atom.
• The use of VSEPR theory to predict the electron domain geometry and the molecular geometry for species with two, three and four electron domains.
• Prediction of bond angles from molecular geometry and presence of non- bonding pairs of electrons.
• Prediction of molecular polarity from bond polarity and molecular geometry.
• Deduction of resonance structures, examples include but are not limited to C₆ H₆,CO₃^2- and O₃.
• Explanation of the properties of giant covalent compounds in terms of their structures

• Does the need for resonance structures decrease the value or validity of Lewis (electron dot) theory? What criteria do we use in assessing the validity of a scientific theory?

Utilization:

• Syllabus and cross-curricular links:
• Option A.7—biodegradability of plastics
• Biology topic 2.3—3-D structure of molecules and relating structure to function

Aims:

• Aim 7: Computer simulations could be used to model VSEPR structures.

• The term “electron domain” should be used in place of “negative charge centre”.
• Electron pairs in a Lewis (electron dot) structure can be shown as dots, crosses, a dash or any combination.
• Allotropes of carbon (diamond, graphite, graphene, C₆₀ buckminsterfullerene) and SiO₂ should be covered.
• Coordinate covalent bonds should be covered.