Radioactive Decay Mcat

Just like planetary orbits, the energy of an electron affects its dynamics. In planetary orbit, a “lower energy” orbit refers to an orbit with a smaller radius that feels a stronger gravitational pull, while a “higher energy” orbit refers to an orbit with a larger radius that feels a weaker gravitational pull. A lower energy orbit is more stable, and a higher energy orbit is less stable. The same principles apply to electron orbit under the Bohr model.

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

  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
    • Definition
    • Variation with group and row
    • Definition
    • Comparative values for some representative elements and important groups

    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
      • Conventions for writing chemical equations
      • Balancing equations, including redox equations
      • Limiting reactants
      • Theoretical yields

      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.

      • Atomic nucleus
      • Electronic structure
      • Periodic table
      • Stoichiometry
      • Balancing chemical equations
      • Redox reactions

      Atomic and Nuclear Physics for the MCAT: Everything You Need to Know

      Learn key MCAT concepts about atomic and nuclear physics, plus practice questions and answers

      

      (Note: This guide is part of our MCAT Physics series.)

      Table of Contents

      Part 1: Introduction to atomic and nuclear physics

      Part 2: Atomic structure

      a) Nuclear structure

      b) Components of the nucleus

      Part 3: Radioactive decay

      a) Alpha decay

      b) Beta decay

      c) Gamma decay

      d) Half-life and exponential decay

      Part 4: High-yield terms and equations

      Part 5: Passage-based questions and answers

      Part 6: Standalone questions and answers
      

      Part 1: Introduction to atomic and nuclear physics

      Atomic and nuclear physics is a wide-ranging topic that covers the structure and behavior of the individual atom. In this guide, we’ll focus on the most important experimental results and the equations that came to describe some of those results.

      On the MCAT, atomic and nuclear physics is a medium-yield topic. Getting these concepts down will help you ace any related questions on the test and might even provide some intuition on chemistry and molecular biology topics, too.

      Similar to our other guides, the most important terms below are in bold font. When you see one, try to define it in your own words and use it to create your own examples. This is a great way to check your understanding, and phrasing things in a way that makes the most sense to you will make studying much easier (and much more effective!) in the long run.

      Let’s get started!

      Part 2: Atomic structure

      An atom is the smallest unit of matter that can comprise a chemical element. For a long time, atoms were thought to be the absolute smallest possible units of matter, until it was discovered that they could be divided into constituent charged particles: namely, protons, neutrons, and electrons, which have positive, neutral, and negative charges, respectively. The magnitude of a single proton and a single electron’s charges is 1.6 x 10 -19 C. Since the charge of a single proton is equal to the charge of a single electron, equal numbers of them create a net charge of zero.

      a) Nuclear structure

      Atoms consist of a central cluster of protons and neutrons surrounded by electrons. The central cluster of protons and neutrons is called the nucleus. Early models of the atom, such as J.J. Thompson’s “plum pudding” model, did not include the nucleus and instead proposed that protons, neutrons, and electrons were evenly distributed throughout the atom. Under the plum pudding model, these three types of particles would be homogeneously mixed about and appear to be present at the same density.

      Rutherford gold foil experiment

      The Rutherford gold foil experiment showed that a dense, positively charged clump of mass must be located at the center of an atom.

      The experimental setup consisted of a very thin sheet of gold foil and a device that would shoot positively charged alpha particles at the foil. (More on alpha particles later, but suffice to say that they are positively charged particles.) If the plum pudding model were correct, the alpha particles would have passed through the foil with no deflection. However, to the researchers’ surprise, a small proportion of the alpha particles experienced a very strong deflection. This suggested the existence of a positively charged nucleus.

      

      The Bohr Model

      By the time the results of the gold foil experiment were known, J.J. Thompson had already discovered the existence of negatively charged electrons. These electrons appeared to exist on the periphery of the atom, but no one had definitively proven their location around the nucleus. This inference was eventually developed into the Bohr Model of the atom, which proposed that electrons orbited the nucleus. This model was analogous to the orbits of planets in our Solar System, where the nucleus was substituted for the sun, electrons for the planets, and electrostatic attraction for gravitational attraction.

      

      Just like planetary orbits, the energy of an electron affects its dynamics. In planetary orbit, a “lower energy” orbit refers to an orbit with a smaller radius that feels a stronger gravitational pull, while a “higher energy” orbit refers to an orbit with a larger radius that feels a weaker gravitational pull. A lower energy orbit is more stable, and a higher energy orbit is less stable. The same principles apply to electron orbit under the Bohr model.

      A key difference from planetary orbits is that the Bohr model states that the possible energy levels are quantized, meaning they are not continuous and can only be certain numbers. The energy levels of the orbiting electrons in a hydrogen atom must follow:

      Note that -13.6 eV (electron volts) is the energy of an electron in the ground state of a hydrogen atom. (In general, this energy depends on the square of the atomic number and the Rydberg constant.)

      This energy can also be thought of as the energy stored in the bond between electron and nucleus. The more negative the energy, the more stable the state. Notice that higher n makes the energy less negative and less stable. When E becomes zero, the electron dissociates. For other elements, a more general equation replaces the numerator with the Rydberg constant (R) and the atom’s atomic number squared (Z 2 ).

      Remember that energy must always be conserved. So, in order for the electron to change energy levels, it needs to either absorb energy or emit energy. The Bohr model specifies that emitted energy is in the form of electromagnetic radiation: most commonly, emitted as visible light. In general, the absorbed energy tends to be electromagnetic radiation, too.

      The type of light absorbed or emitted by the electron is related to the change in energy level because the energy contained in light depends on its frequency. Higher frequency light (or colors that are closer to the color blue in the visible spectrum) has more energy than lower frequency light (like the color red in the visible spectrum). The energy of different frequencies of light is given by:

      Thus, when an electron absorbs blue light, it will jump up more energy levels than an electron that absorbs red light. The relationship between the wavelength of absorbed or emitted light and the change in energy level is given directly by the Rydberg formula: