Chapters 4–5: Atoms, Elements, Molecules & Compounds — Study Materials Summary & Study Notes

🧪 Chapter 5 — Molecules & Compounds

Law of Constant Composition: All samples of a pure substance have the same elemental composition by mass. For example, water is always a 2:1 atom ratio of hydrogen to oxygen in its formula $H_2O$, and carbon dioxide is always $CO_2$. Mixtures (e.g., saltwater) have variable composition.

🧾 Chemical Formulas & Interpretation

A molecule is a particle of two or more nonmetal atoms. A chemical formula shows the types and numbers of atoms (subscripts) in a molecule (e.g., sulfuric acid $H_2SO_4$). Parentheses indicate repeated groups: antifreeze $C_2H_4(OH)_2$ has 2 carbons, 4 hydrogens from the main chain, and 2 OH groups — total H = 6, O = 2.

🧭 Order & Conventions in Formulas

Metals are written first in ionic formulas (e.g., $NaCl$). Nonmetals in molecular formulas are listed according to a convention (example order includes: C, P, N, H, S, I, Br, Cl, O, F). Historical exceptions exist (e.g., $H_2O$, $NaOH$).

⚖️ Classification of Compounds

• Ionic: metal + nonmetal(s) or polyatomic ions (e.g., $NaCl$, $CaBr_2$, $(NH_4)_2SO_4$).
• Molecular: two or more nonmetals (e.g., $CCl_4$).
• Aqueous: dissolved in water, denoted (aq).
• Binary: two elements; ternary: three or more elements.
• Acids: contain hydrogen bonded to nonmetal(s); oxyacids contain hydrogen + oxyanion (e.g., $HNO_3$, $H_2SO_4$).

🔋 Ions & Polyatomic Ions

• Cation: positive ion; metals often lose electrons to form cations (Group IA → +1, Group IIA → +2, Group IIIA → +3). Special single-charge metals: Ag$^+$, Zn$^{2+}$, Cd$^{2+}$.
• Anion: negative ion; nonmetals form anions (Group VIIA → -1, VIA → -2, VA → -3).
• Polyatomic ions: multi-atom ions such as ammonium ($NH_4^+$), sulfate ($SO_4^{2-}$), nitrate ($NO_3^-$), carbonate ($CO_3^{2-}$), hydroxide ($OH^-$), acetate ($C_2H_3O_2^-$), etc. Suffix rules: -ate (more O), -ite (one fewer O).

✏️ Writing Ionic Formulas

Formulas combine cation + anion so overall charge = 0. Use the crossover rule (exchange ion charges as subscripts) but always reduce to lowest whole-number ratio (e.g., $Pb^{4+} + O^{2-} \rightarrow PbO_2$, not $Pb_2O_4$). Polyatomic ions require parentheses when more than one is present (e.g., $Sr(NO_3)_2$).

🔤 Naming Rules — Quick Highlights

• Ionic (one charge metal): cation name + anion stem + -ide (e.g., $MgO$ → magnesium oxide).
• Ionic (variable-charge metal): include Roman numeral for metal charge (e.g., $FeCl_3$ → iron (III) chloride).
• Binary molecular: use Greek prefixes for atom counts; second element ends in -ide (omit mono- on first element) (e.g., $CO$ → carbon monoxide, $PCl_3$ → phosphorus trichloride).
• Acids: Binary acids: hydro- + stem + -ic acid (e.g., $HCl$ (aq) → hydro­chloric acid). Oxyacids: anion ending -ate → -ic acid (e.g., $NO_3^-$ → nitric acid), -ite → -ous acid (e.g., $NO_2^-$ → nitrous acid).

✅ Tips to Remember

• Learn common polyatomic ions and their charges.
• Apply crossover, then reduce subscripts.
• Distinguish between molecular vs ionic by element types (nonmetals vs metal/nonmetal).
• Use Roman numerals when a metal can form multiple positive charges.

⚛️ Chapter 4 — Atoms & Elements

Definition of an atom: The smallest identifiable unit of an element. Roughly 91 naturally occurring elements and ~20 synthetic elements exist.

🏛️ Historical Models

• Dalton: proposed that matter is made of atoms, atoms of an element are identical, and compounds form by whole-number ratios (explains fixed composition like $CO_2$, $H_2O$).
• Thomson: discovered the electron; proposed the plum-pudding model (electrons in a diffuse positive sphere).
• Rutherford: gold-foil experiment showed most alpha particles pass through but some scatter strongly, implying a tiny, dense, positively charged nucleus.

🔬 Subatomic Particles & Atomic Notation

• Proton (p$^+$): positive, mass ≈ $1.67\times10^{-24},\text{g}$.
• Electron (e$^-$): negative, mass ≈ $9.11\times10^{-28},\text{g}$.
• Neutron (n$^0$): neutral, mass ≈ proton.

Atomic number (Z): number of protons. Mass number (A): protons + neutrons. Atomic notation example: sodium with 11 protons and mass 23 is written $^{23}_{11}Na$.

🧬 Isotopes & Atomic Mass

Isotopes are atoms of the same element with different numbers of neutrons (e.g., $^1H$, $^2H$ (deuterium), $^3H$ (tritium)). Atomic mass is a weighted average of naturally occurring isotopes (e.g., Cl average ≈ 35.45 amu from $^{35}Cl$ & $^{37}Cl$ abundances).

🧭 Periodic Table Essentials

• Elements are arranged by increasing atomic number (Moseley). Properties repeat periodically.
• Groups (vertical) share properties (e.g., Group IA = alkali metals, +1 ions). Periods are horizontal rows.
• Metals (lower left): conductive, malleable, form cations. Nonmetals (upper right): poor conductors, form anions. Metalloids show mixed properties (semiconductors like Si).

➕ Ionic Charge Trends

Ionic charges relate to valence electrons: Group IA → +1, IIA → +2, IIIA → +3; Group VIIA nonmetals → -1, VIA → -2, VA → -3. Charges are commonly shown as $Na^+$, $O^{2-}$, etc.

🔑 Study Pointers

• Be able to write atomic notation and identify number of protons/neutrons.
• Practice calculating atomic mass from isotopic abundances.
• Memorize common element symbols and group charges for quick formula writing and naming.