Chemical and Physical Foundations of Biological Systems Section: Content Category 4E

Atoms, nuclear decay, electronic structure, and atomic chemical behavior
Atoms are classified by their atomic number: the number of protons in the atomic nucleus, which also includes neutrons. Chemical interactions between atoms are the result of electrostatic forces involving the electrons and the nuclei. Because neutrons are uncharged, they do not dramatically affect the chemistry of any particular type of atom, but do affect the stability of the nucleus itself.

When a nucleus is unstable, decay results from one of several different processes, which are random, but occur at well-characterized average rates. The products of nuclear decay (alpha, beta, and gamma rays) can interact with living tissue, breaking chemical bonds and ionizing atoms and molecules in the process.

The electronic structure of an atom is responsible for its chemical and physical properties. Only discrete energy levels are allowed for electrons. These levels are described individually by quantum numbers. Since the outermost, or valence, electrons are responsible for the strongest chemical interactions, a description of these electrons alone is a good first approximation to describe the behavior of any particular type of atom.

Mass spectrometry is an analytical tool that allows characterization of atoms or molecules, based on well recognized fragmentation patterns and the charge to mass ratio (m/z) of ions generated in the gas phase.

The content in this category covers atomic structure, nuclear decay, electronic structure, and the periodic nature of atomic chemical behavior. 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
PHY = two-semester sequence of introductory physics
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.

Atomic Nucleus (PHY, GC)

  • Atomic number, atomic weight
  • Neutrons, protons, isotopes
  • Nuclear forces, binding energy
  • Radioactive decay
    • α, β, γ decay
    • Half-life, exponential decay, semi-log plots
  • Mass spectrometer

Electronic Structure (PHY, GC)

  • Orbital structure of hydrogen atom, principal quantum number n, number of electrons per orbital (GC)
  • Ground state, excited states
  • Absorption and emission line spectra
  • Use of Pauli Exclusion Principle
  • Paramagnetism and diamagnetism
  • Conventional notation for electronic structure (GC)
  • Bohr atom
  • Heisenberg Uncertainty Principle
  • Effective nuclear charge (GC)
  • Photoelectric effect

The Periodic Table - Classification of Elements into Groups by Electronic Structure (GC)

  • Alkali metals
  • Alkaline earth metals: their chemical characteristics
  • Halogens: their chemical characteristics
  • Noble gases: their physical and chemical characteristics
  • Transition metals
  • Representative elements
  • Metals and non-metals
  • Oxygen group

The Periodic Table - Variations of Chemical Properties with Group and Row (GC)

  • Valence electrons
  • First and second ionization energy
    • Definition
    • Prediction from electronic structure for elements in different groups or rows
  • Electron affinity
    • Definition
    • Variation with group and row
  • Electronegativity
    • Definition
    • Comparative values for some representative elements and important groups
  • Electron shells and the sizes of atoms
  • Electron shells and the sizes of ions

Stoichiometry (GC)

  • Molecular weight
  • Empirical versus molecular formula
  • Metric units commonly used in the context of chemistry
  • Description of composition by percent mass
  • Mole concept, Avogadro’s number NA
  • Definition of density
  • Oxidation number
    • Common oxidizing and reducing agents
    • Disproportionation reactions
  • Description of reactions by chemical equations
    • Conventions for writing chemical equations
    • Balancing equations, including redox equations
    • Limiting reactants
    • Theoretical yields

Example Question Video

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.