Chemical and Physical Foundations of Biological Systems Section: Content Category 5B

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Nature of molecules and intermolecular interaction

Covalent bonding involves the sharing of electrons between atoms. If the result of such interactions is not a network solid, then the covalently bonded substance will be discrete and molecular.

The shape of molecules can be predicted based on electrostatic principles and quantum mechanics since only two electrons can occupy the same orbital. Bond polarity (both direction and magnitude) can be predicted based on knowledge of the valence electron structure of the constituent atoms. The strength of intermolecular interactions depends on molecular shape and the polarity of the covalent bonds present. The solubility and other physical properties of molecular substances depend on the strength of intermolecular interactions.

The content in this category covers the nature of molecules and includes covalent bonding, molecular structure, nomenclature, and intermolecular interactions. The topics and subtopics are below:

Topic Level Key:

The abbreviations found in parentheses indicate the course(s) in which undergraduate students at many colleges and universities learn about the topics and associated subtopics. The course abbreviations are:

GC = two-semester sequence of general chemistry
OC = two-semester sequence of organic chemistry

Please note topics that appear on multiple content lists will be treated differently. Questions will focus on the topics as they are described in the narrative for the content category.

Covalent Bond (GC)

  • Lewis Electron Dot formulas

    • Resonance structures

    • Formal charge

    • Lewis acids and bases

  • Partial ionic character

    • Role of electronegativity in determining charge distribution

    • Dipole Moment

  • σ and π bonds

    • Hybrid orbitals: sp3, sp2, sp and respective geometries

    • Valence shell electron pair repulsion and the prediction of shapes of molecules (e.g., NH3, H2O, CO2)

    • Structural formulas for molecules involving H, C, N, O, F, S, P, Si, Cl

    • Delocalized electrons and resonance in ions and molecules

  • Multiple bonding

    • Effect on bond length and bond energies

    • Rigidity in molecular structure

  • Stereochemistry of covalently bonded molecules (OC)

    • Isomers

      • Structural isomers

      • Stereoisomers (e.g., diastereomers, enantiomers, cis/trans isomers)

      • Conformational isomers

    • Polarization of light, specific rotation

    • Absolute and relative configuration

      • Conventions for writing R and S forms

      • Conventions for writing E and Z forms

Liquid Phase - Intermolecular Forces (GC)

  • Hydrogen bonding

  • Dipole Interactions

  • Van der Waals’ Forces (London dispersion forces)

Additional Review: Khan Academy MCAT® Collection Tutorials

To support your studies, see the following video tutorials below from the Khan Academy MCAT® Collection. The videos and associated questions were created by the Khan Academy in collaboration with the AAMC and the Robert Wood Johnson Foundation.

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