Chapter 2 Notes: The Chemical Context of Life
Concept 2.1: Matter consists of chemical elements in pure form and in combinations called compounds
Organisms are matter.
Element: pure substance of one atom type; cannot be broken down (e.g., O, C, H, N).
Compound: fixed ratio of elements, with unique emergent properties (e.g., NaCl).
Life requires ~25 elements: macroelements (O, C, H, N, Ca, P) and trace elements (Fe, I).
Concept 2.2: An element’s properties depend on the structure of its atoms
Atom: smallest unit retaining element properties (protons, neutrons in nucleus; electrons in cloud).
Atomic number (Z): number of protons.
Mass number (A): protons + neutrons.
Isotopes: same element, different neutron numbers (radioactive isotopes used as tracers, for dating).
Electrons exist in discrete energy levels (shells) and orbitals (3D regions).
Valence electrons (outermost) determine chemical behavior; octet rule (8 electrons for stability, 2 for first shell).
Concept 2.3: Bonding and the Formation of Molecules
Atoms form chemical bonds to achieve stable electron configurations.
Covalent bonds: share electrons.
Electronegativity: atom's electron attraction.
Nonpolar covalent: equal sharing.
Polar covalent: unequal sharing (e.g., H2O).
Ionic bonds: transfer electrons, forming ions (cations +, anions -) (e.g., NaCl).
Weak chemical bonds (e.g., hydrogen bonds, Van der Waals interactions) are crucial for molecular shape and biological function.
Molecular shape dictates how molecules interact (e.g., drug binding).
Concept 2.4: Chemical Reactions Make and Break Chemical Bonds
Chemical reactions rearrange matter (reactants to products).
Example: Photosynthesis ($$6 CO2 + 6 H2O \rightarrow C6H{12}O_6 + 6 O