Notes: Chemical Elements, Molecules, Water, and Acids & Bases

2.1 Chemical Elements

  • What is matter?
    • Anything that has mass.
    • Anything that occupies space.
    • Exists in four states: solid, liquid, gas, plasma.
    • Composed of elements.
  • Physical states (order of increasing energy):
    • Solid → Liquid → Gas → Plasma
  • An element is a substance composed of one type of atom.
    • Elements cannot be broken down into substances with different properties.
    • Six elements make up 95% of the bodyweight of organisms (acronym CHONPS): Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Sulfur (S).
  • An atom is the smallest part of an element that displays the property of the element.
    • Subatomic particles:
    • Protons; positive charge
    • Neutrons; neutral/no charge
    • Electrons; negative charge
  • Atomic symbol, atomic number, mass number, and atomic mass:
    • Atomic symbol: a unique one- or two-letter designation for each element (e.g., H = hydrogen, Na = sodium, C = carbon).
    • Atomic number = number of protons in each atom of an element.
    • Mass number = sum of protons and neutrons in the nucleus.
    • Atomic mass = average mass for all isotopes of that atom; approximately equal to the mass number.
  • Arrangement in the periodic table:
    • Horizontally in rows called periods (increasing atomic number).
    • Vertical columns are called groups.
  • Isotopes:
    • Atoms of the same element that differ in the number of neutrons.
    • Radioactive isotopes emit energy in the form of rays and subatomic particles.
    • Carbon-14 is an example of a radioactive isotope; has been used to examine reactions in photosynthesis.
    • Over time, Carbon-14 changes to Nitrogen-14 through radioactive decay.
  • Valence shell:
    • Outermost energy shell of an atom.
    • Determines many of an atom’s chemical properties.
  • An anatomical note from the transcript (example items listed under 2.1): larynx, thyroid gland, trachea.
  • Key terms to remember:
    • Element, atom, atomic symbol, atomic number, mass number, atomic mass, isotopes, radioactive decay, valence shell.

2.2 Molecules and Compounds

  • A molecule is two or more of the same type of atoms bonded together.
  • A compound is a molecule containing at least two different elements bonded together.
  • A formula tells the number of each kind of atom in a molecule.
  • Bonds store energy and are caused by interactions between electrons in outermost energy shells.
  • The process of bond formation is a chemical reaction.
  • An ion is an atom that has lost or gained an electron.
  • Ionic bonds:
    • Form when electrons are transferred from one atom to another.
    • Oppositely charged ions are attracted to each other.
  • Covalent bonds:
    • Form when two atoms share electrons so each atom achieves an octet in the outer shell.
    • Nonpolar covalent bond: electrons are shared equally between atoms.
    • Polar covalent bond: electrons are shared unequally.
  • Electronegativity:
    • The ability of an atom to attract electrons toward itself in a chemical bond.

2.3 Chemistry of Water

  • Water is a polar molecule.
  • A hydrogen bond is a weak attraction between a slightly positive hydrogen atom and a slightly negative atom.
  • Properties of water:
    • Liquid at temperatures found on Earth’s surface.
    • High heat capacity.
    • High heat of evaporation.
    • Solvent.
    • Cohesive.
    • Adhesive.
    • Water is less dense as a solid than as a liquid (ice floats).

2.4 Acids and Bases

  • Potential of Hydrogen (pH) is a measure of hydrogen ion concentration in a solution.
    • Indicates the acidity or alkalinity of a solution.
    • Logarithmic scale.
    • Each unit change in pH represents a 10-fold change in H⁺ concentration.
    • Example relationships:
    • ext{pH} = -\\log_{10}[H^+]
    • A change of one pH unit: [H^+]{pH=4} = 10^{-4}, \ [H^+]{pH=5} = 10^{-5} \ \ \frac{[H^+]{pH=4}}{[H^+]{pH=5}} = 10
  • Acids:
    • Dissociate in water and can donate hydrogen ions (protons).
    • Lower the pH of a solution.
  • Bases:
    • Can accept hydrogen ions (protons) and release hydroxide ions.
    • Increases the pH of a solution.
  • Buffer:
    • A substance that resists changes in pH when small amounts of acids or bases are added.
  • Blood pH:
    • Normally around 7.4 (slightly basic).
    • If pH drops below 7.0, acidosis results.
    • If pH rises above 7.8, alkalosis results.
  • Homeostasis:
    • The body has built-in mechanisms to prevent pH changes.
    • Carbonic acid (H₂CO₃) is involved in buffering (as part of the acid-base balance context).