Biology/Chemistry Notes: Atoms, Bonding, and Water

iClicker troubleshooting and class logistics

• If iClicker has issues, visit the help pages and contact support. If the issue isn’t resolved, involve the instructor; there are things the instructor can do that students cannot.
• The campus has many classes and a lucrative contract with iClicker; this incentivizes a seamless experience for students.
• If problems persist after students troubleshoot, inform the instructor so adjustments can be made to ensure students have the opportunity to complete the tasks.
• Academic integrity website was reshuffled; some questions didn’t make sense initially; the instructor indicates it should be working now.
• If you still have problems, notify the instructor and they will make necessary adjustments to provide opportunities to complete the work.
• Regular schedule: on Tuesdays and Thursdays there is a homework assignment due; you can finish it before class; double-check your assignments.
• Registration and account setup: if prompted for a join code, identify quickly and get to class. If you have a physical clicker, remember to associate it so we can generate questions.
• The instructor says many people on campus have similar issues and the system is designed to be user-friendly, but if you encounter problems, you should try on your end first, then report.

Atomic structure and basic hierarchy

• Atoms are described as the smallest stable components of matter; cannot be broken down into stable components (within this simplified view).
• You can break matter down further, but subatomic particles are often considered unstable in this context.
• Hierarchy (as presented in the lecture): from a cell to organelles that make up cells, to molecules that help make up organelles, and down to atoms (note: the order stated here is a simplified view for the lecture; scientifically, the typical hierarchy is atoms → molecules → organelles → cells).
• In introductory context, subatomic particles are sometimes ignored because they are not the most stable; you may encounter these in physics or chemistry courses later.
• The basic atomic structure introduced is the Bohr model: nucleus in the center, electrons orbiting around it; example shown with one electron around the nucleus.
• The nucleus contains protons (positive) and neutrons (neutral). Protons and neutrons together have a mass unit often referred to as a Dalton (Da).
• Dalton is named after John Dalton (the “Dalton” unit). The instructor makes a playful aside about a historical reference.
• Outside the nucleus, electrons occupy orbitals; the first orbital holds a maximum of two electrons; higher energy levels tend to hold up to eight electrons (octet rule in many contexts).
• This depiction is not to scale; atoms are mostly empty space.
• When discussing atoms and their behavior, we focus on the number of protons, neutrons, and electrons to determine identity and chemistry (bonds, reactivity).

Atomic number, mass, and neutrons

• Atomic number Z: the number of protons in an atom; defines the element type.
• Atomic mass (mass number) A: roughly the sum of protons and neutrons in the nucleus; in many contexts treated as approximately equal to the number of nucleons.
• How to get neutrons: N = A − Z.
• Isotopes differ in the number of neutrons but have the same Z.
• Mass units: protons and neutrons contribute to mass; electrons contribute negligibly to atomic mass.
• The nucleus contains protons (positive charge) and neutrons (neutral charge); electrons are negatively charged and reside in orbitals around the nucleus.
• The mass of a proton or neutron is about 1 Da (1 atomic mass unit).

Electron shells, valence electrons, and the octet rule

• First electron shell (closest to the nucleus) can hold up to 2 electrons.
• All subsequent shells can hold up to 8 electrons (octet rule in many teaching models).
• Valence electrons: electrons in the outermost shell; these electrons largely determine reactivity and bonding behavior.
• Helium is special: it has only two electrons, filling its first energy level; it is nonreactive because its outer shell is complete with two electrons.
• When electrons are excited to higher energy levels, energy is absorbed; when electrons drop to lower energy levels, energy is released.
• In simple drawings, electrons are often shown in pairs; this is a simplification.
• Most of the atom is empty space; the visual is a representational tool, not a literal map of electron positions.

Atomic structure details and mass calculations

• Inside the nucleus: protons (+) and neutrons (no charge); outside: electrons (−).
• The number of protons defines the element via atomic number Z (e.g., carbon Z=6, sodium Z=11, chlorine Z=17).
• Mass of the nucleus is largely due to protons and neutrons; electrons contribute very little to atomic mass.
• To estimate neutrons: N = A − Z, where A is the mass number (total protons and neutrons).
• The Bohr model helps explain electron energy levels and transitions, though modern quantum models are more accurate for complex atoms.

Bonding: ionic, covalent, and hydrogen bonds

• Ionic bonds: result from electrostatic attraction between oppositely charged ions (e.g., Na+ and Cl−).
  • In a lattice, ions arrange in an organized, crystalline structure due to charge interactions.
  • In biology, ionic interactions can be significant but are often moderated by the aqueous environment and solvation.
• Covalent bonds: strongest bonds in biological contexts; electrons are shared between two atoms.
  • A single bond involves sharing one pair of electrons; a double bond shares two pairs; a triple bond shares three pairs.
  • Covalent bonds are strong because electrons are shared, and the bond strength increases with the number of shared electron pairs.
• Hydrogen bonds: weaker than covalent and ionic bonds; occur when a hydrogen is attracted to a highly electronegative atom (often O, N, or F).
  • In biomolecules, hydrogen bonding contributes to structure and dynamics; they are crucial for the properties of water and many biological structures.
• In aqueous environments, ionic bonds are not as dominant as in a vacuum; solvation and competition with water molecules can weaken or screen ionic interactions.
• Polar vs nonpolar covalent bonds:
  • If electrons are shared unequally due to differences in electronegativity, the bond is polar covalent with partial charges (δ+ and δ−).
  • If electrons are shared more evenly, the bond is nonpolar covalent.
  • Water (H2O) is a polar molecule because of the electronegativity difference between oxygen and hydrogen, leading to partial charges and hydrogen bonding.
• Electronegativity influences bond polarity and molecular geometry, which in turn affects reactivity and solubility.

Water: polarity, hydrogen bonding, and emergent properties

• Water is a polar molecule: O is more electronegative than H, leading to partial negative charge on O and partial positive charges on H.
• Hydrogen bonds occur between water molecules and between water and other polar molecules; these bonds are not as strong as covalent bonds but are numerous and collectively significant.
• Emergent properties of water (as discussed in the lecture):
  • Cohesion: water molecules stick to each other via hydrogen bonds, contributing to surface tension.
  • Adhesion: water molecules stick to other surfaces (e.g., glass), contributing to capillary action.
  • Moderation of temperature: water has a high specific heat capacity and a high heat of vaporization, allowing it to moderate temperatures in organisms and environments.
  • Expansion upon freezing: ice expands and becomes less dense than liquid water, which has ecological implications (e.g., ponds not freezing solid from bottom up).
  • Water as a versatile solvent: dissolves many substances due to its polarity and hydrogen-bonding capability; this underpins biochemical reactions and transport in organisms.
• The relative strengths of bonds in biology (in aqueous environments): covalent bonds are strongest, ionic bonds are intermediate (often weakened by water), hydrogen bonds are weaker but numerous and functionally important.
• Surface tension arises from cohesive forces at the air–water interface, enabling phenomena like water striders to move on water surfaces.

Water’s role as a solvent and the pH concept

• Water acts as a solvent for many biological processes; its polarity allows it to dissolve ionic compounds and many polar molecules.
• pH basics:
  • pH is a measure of hydrogen ion concentration: $ext{pH} = -\log_{10}[H^+]$
  • Neutral pH (at standard conditions) corresponds to $[H^+] = [OH^-] = 10^{-7} ext{ M}$, i.e., pH = 7.
  • The scale is logarithmic: a change of 1 pH unit corresponds to a tenfold change in hydrogen ion concentration. For example, moving from pH 7 to pH 6 increases [H+] by a factor of 10; from pH 7 to pH 9 decreases to 1/100 of the original hydrogen ion concentration.
  • The lecture notes mention a five-order-of-magnitude difference (10^5) for a move from pH 7 to pH 13; actually, the difference from 7 to 13 is 6 pH units, corresponding to a factor of 10^6 in [H+]. In the notes, this is stated as five orders of magnitude, per the instructor’s phrasing, which is an approximation used in that moment.
• Water autoprotolysis (autoionization) and the pH scale connect to the concept of pH in biology, where maintaining near-neutral pH is often essential for enzyme activity and cellular function.
• Ionization of water and acid–base concepts can be summarized by the equilibrium:
  • $ext{H}_2 ext{O} <br /> ightleftharpoons ext{H}^+ + ext{OH}^-$
  • The equilibrium constant is $K_w = [ ext{H}^+][ ext{OH}^-] \approx 1.0 imes 10^{-14}$ at 25°C.
• Practical note (biological relevance): the presence of salts and buffers in biological systems influences ionic strength and pH, affecting protein structure and function.

Quick connections and practical implications

• The iClicker/logistics discussion ties into practical campus life aspects that can influence learning, attendance, and timely homework submission; staying engaged and solving tech issues promptly supports consistent study and comprehension.
• Foundations of atomic structure and bonding underpin all of biochemistry and molecular biology: how atoms bond, form molecules, and give rise to macromolecules like proteins and nucleic acids.
• Water’s properties explain why life exists where it does: temperature moderation by oceans/climates, solvent capabilities for biochemical reactions, and the role of hydrogen bonds in biomolecular structure (e.g., DNA base pairing, protein folding).
• Ethical and practical implications: understanding that bond strengths and solvent effects can influence drug design, nutrient transport, and environmental factors (e.g., how water’s properties affect climate stability and ecosystem health).

Study cues and exam-oriented reminders

• Expect questions contrasting bonding types: covalent vs ionic vs hydrogen bonds; polar vs nonpolar covalent bonds; and the conditions under which each type dominates.
• Be prepared to explain why covalent bonds are strongest in biology, especially in aqueous environments, and why ionic bonds can be weaker due to solvation.
• Be able to differentiate adhesion vs cohesion and relate both to water’s surface tension and behavior in real-world contexts (e.g., droplets, capillary action).
• Understand the octet rule and valence electrons, including why helium is special and how lone pairs influence reactivity and molecular geometry.
• Be comfortable with basic pH calculations and the concept of pH as a log scale; know the equations for pH, hydrogen ion concentration, and water autoionization.

Final note from the instructor’s remarks (classroom context)

• The acoustics in the classroom are very good; distractions can affect learning, so paying attention is important.
• The instructor emphasizes care about students’ grades and encourages focusing on the material even when the class has distractions.
• The upcoming theme includes a deeper dive into adhesion vs cohesion and other emergent properties; there may be test questions distinguishing these terms, so take notes on those distinctions.
• Assignment schedule and reading: there are upcoming tasks; start with lesson four readings and ensure you understand the concepts of adhesion, cohesion, and the properties of water discussed above.