The Chemistry of Life

Properties of the Chemical Elements

Key Questions About Life and Chemistry

• Why is too much sodium harmful?
• Why does an iron deficiency cause anemia?
• Why does a pH imbalance make some drugs less effective?
• How can radiation cause cancer as well as cure it?

The Chemical Elements

• Simplest form of matter that can maintain unique chemical properties.
• Example: Water (H_2O) is a molecule made up of the elements (atoms) hydrogen and oxygen; water has its own properties; hydrogen and oxygen also have unique properties on their own.

The Chemical Elements: The Periodic Table

• Each element is assigned a numerical identifier called the atomic number, based on the number of protons in its nucleus.
• This numerical assignment also determines the order of these elements in the periodic table.
• The atomic weight/mass is the number of protons plus neutrons in the atom.

Molecular Weight (MW)

• MW of compound = sum of atomic weights of atoms.
• Calculate: MW of glucose (C6H{12}O_6)
  • 6 C atoms x 12 amu each = 72 amu
  • 12 H atoms x 1 amu each = 12 amu
  • 6 O atoms x 16 amu each = 96 amu
  • Molecular weight (MW) = 180 amu

Major Elements

• Of the 91 elements, 24 are physiologically relevant to humans.
• Six of them—carbon, hydrogen, oxygen, nitrogen, calcium, and phosphorus—account for more than 98% of each organism.

Trace Elements

• Found in minute quantities in humans (account for 0.7% body weight).
• Play important roles in physiology.

Minerals

• Extracted from the soil by plants, transported through the food chain to humans.
• Constitute ~4% of human body weight and contribute greatly to body structure (e.g., teeth and bones contain calcium, phosphate, magnesium, fluoride, and sulfate ions).
• Enable enzymes and other molecules to function (act as cofactors); for example, iodine is a component of thyroid hormone; iron is an integral component of hemoglobin, etc.

The Atom

• An atom is the smallest particle of an element.
• Nucleus = center of atom.
  • Protons: + charge.
  • Neutrons: no charge.
• Electron shells surround the nucleus.
  • Electrons: negative charge.
  • Valence electrons in the outermost shell.
  • Interact with other atoms.
  • Determine chemical behavior.

Planetary Models of Elements

• Diagrams illustrating electron arrangement in shells around the nucleus. Examples given are Carbon (C), Nitrogen (N), Sodium (Na), and Potassium (K).
• Carbon (C): Atomic number = 6, Atomic mass = 12, 6 protons, 6 neutrons.
• Nitrogen (N): Atomic number = 7, Atomic mass = 14, 7 protons, 7 neutrons.
• Sodium (Na): Atomic number = 11, Atomic mass = 23, 11 protons, 12 neutrons.
• Potassium (K): Atomic number = 19, Atomic mass = 39, 19 protons, 20 neutrons.

In-Class Activity

• Draw the corresponding planetary models (Lewis structures) for elements with atomic numbers: 11, 17, 39, 18, 8, 25, 43.

Radioisotopes and Radioactivity

• Isotopes
  • Different forms of a given element (differ in number of neutrons and also in atomic mass).
  • Same chemical behavior, differ in physical behavior.
  • Breakdown gives off radiation as they achieve a more stable form.
• Radioisotopes
  • Unstable isotopes.
  • Every element has at least one radioisotope.
• Radioactivity
  • Radioisotopes decay to stable isotopes releasing radiation.
  • We are all mildly radioactive.

Marie Curie

• Coined the term radioactivity.
• Discovered radioactivity of radium.
• Trained physicians in the use of X-rays and also radiation therapy as cancer treatment.
• First woman in the world to receive a PhD.
• First woman to receive the Nobel Prize (1903) and the first person to win twice! (again in 1911).
• Died of radiation poisoning at 67.

Ions and Ionization

• Ions - carry a charge due to an unequal number of protons and electrons.
• Ionization = transfer of electrons from one atom to another (to achieve stability of valence shell).

Ionizing Radiation

• High-energy radiation that releases electrons from atoms, thus creating ions.
• Destroys molecules and produces dangerous free radicals.
• Sources include UV light and X-rays, nuclear decay (\alpha, \beta , \gamma ).
  • \alpha particles (dangerous if inside the body)
    • 2 protons + 2 neutrons; can’t penetrate skin (radon, polonium).
  • \beta particles (dangerous if inside the body)
    • Free electron; penetrates skin a few millimeters (^{32}P, tritium, ^{14}C).
  • \gamma particles (emitted from uranium and plutonium)
    • Highly penetrable; very dangerous.

Ionizing Radiation (cont’d)

• Physical half-life of radioisotopes
  • Time needed for 50% to decay.
  • Nuclear power plants create radioisotopes.
• Biological half-life of radioisotopes
  • Time for 50% to disappear from the body.
  • Dependent on rate of decay and physiological clearance (i.e., excretion).

Free Radicals

• Particles with an odd number of electrons.
• Produced by:
  • Normal metabolic reactions, radiation, chemicals.
• Cause tissue damage:
  • Reactions that destroy molecules.
  • Cancer, death of heart tissue, and aging.
• Antioxidants:
  • Neutralize free radicals.
  • Provided by diets rich in vitamin E, carotenoids, vitamin C, to name a few.
  • Deficiencies have been linked to many disorders.