Lecture 2: Atoms, Isotopes, and PET Imaging

Introduction: Why learn basic chemistry for biology

• PET brain scans show activity levels using colors: red = high activity; dark colors (blue, purple, black) = little to no activity.
• Scans come from different patients; normal brain vs. mild cognitive impairment vs. Alzheimer's disease show increasing darkness in the brain regions.
• PET imaging is common in medicine and relies on atoms and radioactivity; understanding atoms helps explain how PET works.

Elements, atoms, and the identity of matter in biology

• Living organisms are made of elements, which are pure substances composed of the same type of atom.
• Elements differ from one another because they have unique chemical and physical properties.
• Elements vs. compounds:
  • An element is made of one type of atom (e.g., sodium or chlorine in their pure forms).
  • A compound is made of two or more elements bonded together (e.g., NaCl, which is salt).
• Examples of elements commonly discussed in biology:
  • Carbon (C), Oxygen (O), Nitrogen (N), Hydrogen (H), etc.
• Simple illustration to contrast reactivity and state:
  • Sodium (Na): solid at room temperature, soft, highly reactive, especially with water.
  • Chlorine (Cl): gas, highly reactive and caustic when dissolved in water.
• When sodium and chlorine react, they form sodium chloride (NaCl), a compound that is relatively harmless in small amounts and essential in our diet.
• Major elements by mass in the human body:
  • The top four: Oxygen (O), Carbon (C), Hydrogen (H), Nitrogen (N) together account for about 96.3% of body mass.
  • Other important elements (e.g., calcium, phosphorus, potassium) account for about 3.7%.
• Trace elements: present in very small quantities but are essential; high levels can be toxic.

Major contributor to body mass: oxygen, carbon, hydrogen, nitrogen

• The four most abundant elements by mass: $ext{O}, ext{C}, ext{H}, ext{N}$
• Oxygen alone makes up more than half of body mass; often cited as about 65% of body mass.
• Where does this oxygen mass come from?
  • While we breathe O₂ (a gaseous molecule), most body mass comes from the oxygen atoms bound in water, $ext{H}_2 ext{O}$, which forms a large fraction of bodily mass.
  • Water molecules contain one oxygen atom per molecule, and water molecules are abundant in tissues.
• Oxygen exists as ( ext{O}_2 ) in the air, but the majority of body mass is from oxygen atoms in water and other compounds, not from inhaled O₂ gas itself.

Atomic structure: protons, neutrons, and electrons

• Atoms are extremely small and are composed of three types of particles:
  • Protons: positively charged ((+))
  • Neutrons: neutral (no charge)
  • Electrons: negatively charged ((-))
• The nucleus contains protons and neutrons; electrons occupy regions outside the nucleus called electron shells (or orbitals in some pictures).
• Mass and charge distribution:
  • Protons and neutrons contribute almost all of an atom’s mass.
  • Each proton and neutron has mass ~1 Dalton (1 amu).
  • Electrons have very small mass; for most mass calculations in introductory chemistry we treat electron mass as negligible.
• Common visual models:
  • Left illustration: electrons occupy a cloud-like region (orbitals) around the nucleus.
  • Right illustration: electrons occupying distinct electron shells (concentric circles).
• Mass of the nucleus:
  • Protons and neutrons each have mass ~1 amu; the total nucleus mass is approximately the sum of protons and neutrons:
  • For an atom with two protons and two neutrons, the mass ≈ 4 amu.

Atomic number, mass number, and the identity of an element

• Definitions:
  • Atomic number ((Z)) = number of protons in the nucleus; this defines the element.
  • Mass number ((A)) = total number of protons and neutrons in the nucleus; (A = Z + N).
  • Neutron number ((N)) = (A - Z).
• In a simplified periodic table example, bottom number = atomic number ((Z)); top number = mass number ((A)) in the shown boxes.
• Example: Helium box
  • Protons: 2, Neutrons: 2 → Mass number (A = 2 + 2 = 4).
  • Atomic number (Z = 2) (two protons define helium).
• The identity of an element is determined by its atomic number (Z); no two elements share the same (Z).
• Important note on mass number representations:
  • In real periodic tables, atomic masses are not whole numbers (they’re averages over isotopes), e.g., carbon has an average atomic mass about $12.011$, reflecting isotopic abundances.

Isotopes: atoms of the same element with different masses

• Isotopes are atoms of the same element (same (Z)) but with different mass numbers (different numbers of neutrons).
• Example: Carbon isotopes
  • (^{12}\text{C}): 6 protons, 6 neutrons; mass number (A = 12).
  • (^{13}\text{C}): 6 protons, 7 neutrons; mass number (A = 13).
  • (^{14}\text{C}): 6 protons, 8 neutrons; mass number (A = 14); radioactive (unstable).
• Abundances for carbon isotopes (illustrative):
  • Carbon-12: ~98–99% of natural carbon.
  • Carbon-13: ~1%.
  • Carbon-14: trace amounts (radioactive).
• Some hydrogen isotopes:
  • Protium: (^{1}\text{H}) → 1 proton, 0 neutrons; mass number 1; most abundant.
  • Deuterium: (^{2}\text{H}) (also called D) → 1 proton, 1 neutron; mass number 2; less than a percent.
  • Tritium: (^{3}\text{H}) (also called T) → 1 proton, 2 neutrons; mass number 3; rare.
• Isotopes can be stable or radioactive:
  • Most isotopes are stable, but some are radioactive (unstable) and decay over time.
• Radioactive isotopes and real-world usefulness:
  • Some isotopes are unstable and decay, releasing particles and energy (radioactivity).
  • Examples: (^{18}\text{F}) (fluorine-18), (^{11}\text{C}) (carbon-11), and (^{238}\text{U}) (uranium-238).
  • Radioactive isotopes have medical uses, such as PET imaging and tissue localization.

Radioactive isotopes in medicine and PET imaging

• Positron emission tomography (PET) uses radioactive isotopes to visualize metabolic activity.
• A common PET tracer is fluorodeoxyglucose (FDG), a glucose analog labeled with radioactive fluorine: fluorine-18.
  • FDG is produced by attaching radioactive (^{18}\text{F}) to glucose, forming (^{18}\text{F} ext{-FDG}
    ").
  • Glucose is a critical energy source for cells; cancer cells and highly active brain cells have high glucose uptake/metabolism.
• How FDG-based PET works:
  • Inject FDG into the body; cells with high metabolic rates take up more FDG.
  • The radioactive decay of (^{18}\text{F}) emits detectable radiation that the PET scanner records.
  • The resulting image maps regions of high uptake (hot spots), highlighting active brain areas or cancerous tissue.
• Other PET isotopes mentioned:
  • (^{11}\text{C}) (carbon-11) as another radioactive tracer.
  • Uranium-238 is cited as an example of a radioactive isotope used in other contexts (e.g., weapons and reactors), illustrating the diversity of radioactive materials.
• Why these isotopes are useful in medicine:
  • They help identify locations of high metabolic activity, such as tumors or regions of the brain with abnormal activity.
  • They enable non-invasive imaging and can inform diagnosis and treatment planning.

Summary and connections

• Key ideas:
  • Elements are defined by the number of protons (atomic number, (Z)); atoms of the same element can have varying numbers of neutrons (isotopes) that change the mass number ((A)).
  • The mass number is the sum of protons and neutrons: $A = Z + N$; hence $N = A - Z$.
  • The bulk of the body’s mass comes from oxygen atoms bound in water and other molecules; water is represented as $ext{H}<em>2 ext{O}$, and oxygen gas is $ext{O}</em>2$ in the air.
  • Electron mass is negligible for most mass calculations; the nucleus (protons and neutrons) provides most of the atom’s mass.
  • Isotopes vary in stability: some are stable, others are radioactive and decay, releasing energy and particles.
  • PET imaging relies on radioactive isotopes (e.g., $^{18}\text{F}$ in FDG) to visualize metabolic activity in tissues.
• Connections to biology and medicine:
  • Understanding atoms and isotopes underpins interpretation of brain PET scans and cancer imaging.
  • Knowledge of isotopes explains why certain tracers accumulate in rapidly metabolizing tissues and how activity maps are generated.
• Ethical and practical implications:
  • Use of radioactive materials requires safety considerations, dose management, and regulatory oversight to minimize exposure.
  • PET imaging provides valuable diagnostic information while balancing risks associated with radioactive exposure.

Quick practice checks (conceptual)

• If an atom has 6 protons and 7 neutrons, what is its mass number A and which element is it likely to be? Answer: A = 6 + 7 = 13; element with atomic number Z = 6 is carbon.
• In carbon, what distinguishes isotopes like carbon-12 and carbon-13? Answer: They share Z = 6 (same element) but have different A (12 vs 13) due to different numbers of neutrons.
• What is the mass number of the fluorine atom shown with 9 protons and 10 neutrons? Answer: A = 9 + 10 = 19.
• Why is it useful to label glucose with a radioactive isotope for PET imaging? Answer: Metabolically active tissues take up more glucose; radioactive decay of the tracer marks those regions so the PET scanner can visualize them.

Note on symbols and notation

• Atomic number: (Z) (number of protons). Mass number: (A) (protons + neutrons).
• Isotopes: same (Z), different (A).
• Common symbols:
  • Water: $\text{H}_2\text{O}$
  • Oxygen gas: $\text{O}_2$
  • Isotope examples: $^{12}\text{C},\;^{13}\text{C},\;^{14}\text{C},\;^{1}\text{H},\;^{2}\text{H},\;^{3}\text{H}$
• Abundances (illustrative): carbon-12 ~98–99%, carbon-13 ~1%, carbon-14 trace; hydrogen-1 is the most abundant hydrogen isotope.