Comprehensive Study Notes — Atoms, Elements, Molecules & Compounds Summary & Study Notes

🧪 Law & Formulas

Law of Constant Composition: All pure substances have a constant composition — the same elements in the same percent (ratio) by mass. Examples: $H_2O$ is always a 2:1 H:O ratio; $CO_2$ is always 1 C : 2 O. Mixtures (e.g., saltwater) have variable composition.

Chemical formula: Expresses the number and types (and numbers) of atoms in a particle. Example: sulfuric acid is $H_2SO_4$. Subscripts indicate count; a single atom of a type omits the subscript "1".

🔍 Interpreting & Writing Formulas

Parentheses clarify composition and show grouping with subscripts: antifreeze $C_2H_4(OH)_2$ contains 2 C, 4 H, and 2 OH groups → total H = 6, O = 2. Order of elements: metals first in ionic formulas ($NaCl$). For molecular (nonmetals) follow conventional order (e.g., $CO_2$, and an informational exception $H_2O$, $NaOH$). When using atomic symbols in text or formulas, place counts in math mode: e.g., $C_6H_6$.

🧩 Classification of Compounds

• Ionic: cation + anion, typically metal + nonmetal(s) (e.g., $NaCl$, $CaBr_2$, $KMnO_4$, $(NH_4)_2SO_4$).
• Molecular (covalent): two or more nonmetals (e.g., $CCl_4$).
• Aqueous: dissolved in water (label as (aq)).
• Binary: contains two elements. Ternary: three or more elements.
• Acid: contains hydrogen combined with nonmetal(s); oxyacid: contains hydrogen + polyatomic oxyanion (e.g., $HNO_3$, $H_2SO_4$).

⚛️ Ions: Types & Naming

Cation / Anion: positive vs negative ions. Monoatomic ions come from single atoms (e.g., $Na^+$, $Cl^-$, $O^{2-}$). Polyatomic ions contain multiple atoms (e.g., $OH^-$, $SO_4^{2-}$, $NH_4^+$).

Common monoatomic ion charges: Group IA = $+1$, Group IIA = $+2$, Group IIIA = $+3$ (e.g., $Al^{3+}$). Many d-block metals form multiple charges — use the Stock system with Roman numerals (e.g., CoCl$_3$ = cobalt(III) chloride).

Polyatomic ion patterns: most oxyanions end in -ate (higher O count) and -ite (one fewer O). Examples: carbonate $CO_3^{2-}$, sulfate $SO_4^{2-}$, nitrate $NO_3^{-}$, phosphate $PO_4^{3-}$, hydroxide $OH^-$, ammonium $NH_4^+$.

🧾 Writing Ionic Formulas & Crossover Rule

Ionic compounds must be electrically neutral. Techniques:

• If charges are equal and opposite, use one of each: $Na^+ + Cl^- \to NaCl$; $Mg^{2+}+O^{2-}\to MgO$.
• If charges differ, use the crossover rule: swap magnitudes (without signs) to become subscripts (and then reduce to lowest whole-number ratio). Example: $Al^{3+}$ + $SO_4^{2-}$ gives $Al_2(SO_4)_3$ (use parentheses for multiple polyatomic ions).
• Always reduce to the lowest whole-number ratio: $Pb^{4+} + O^{2-}$ becomes $PbO_2$ (not $Pb_2O_4$).

🏷️ Naming Ionic & Molecular Compounds

Ionic (single-charge metals): cation name + anion name (with -ide suffix for simple anions). Example: $MgO$ = magnesium oxide.

Ionic (variable-charge metals): cation name with Roman numeral indicating charge + anion name. Example: $Fe(OH)_3$ = iron(III) hydroxide.

Binary molecular (two nonmetals): use Greek prefixes to indicate atom counts (mono-, di-, tri-, tetra-, penta-, hexa-, etc.). Omit the prefix mono- on the first element (exception: $CO$ = carbon monoxide). Example: $PCl_3$ = phosphorus trichloride; $Cl_2O_7$ = dichlorine heptaoxide.

🧫 Acids Naming Rules

Acids are named based on the anion produced when dissolved in water:

• Binary acids: $H$ + nonmetal → prefix hydro- + nonmetal stem + -ic acid (e.g., $HCl$ (aq) = hydrobromic acid for $HBr$ (aq)).
• Oxyacids: anion ending in -ate → acid name ends in -ic (e.g., $NO_3^-$ → nitric acid $HNO_3$). Anion ending in -ite → acid name ends in -ous (e.g., $NO_2^-$ → nitrous acid $HNO_2$).

✅ Quick Problem Strategies

• Determine whether a compound is ionic or molecular by checking metal vs nonmetal content.
• Use group positions to predict common ionic charges (e.g., Group VIIA → $-1$, Group VIA → $-2$).
• For naming, identify ions first, then assemble cation + anion name; for molecular compounds, use prefixes for counts.

🧬 What is an Atom?

Atom: the smallest identifiable unit of an element. Natural elements ≈ 91; ~20 synthetic elements exist. Atoms are composed of subatomic particles: protons, neutrons, and electrons.

🏛️ Historical Models

• Democritus: early idea of indivisible particles, "atomos."
• Dalton (early 19th c.): atoms are indivisible units; compounds form from atoms in simple whole-number ratios (explains laws like definite composition). Example predictions: $CO_2$, $H_2O$, and methane $CH_4$.
• Thomson: discovered the electron; proposed the "plum pudding" model where electrons are embedded in a positive sphere.
• Rutherford: gold-foil experiment showed most of the atom is empty space with a dense, positively charged nucleus that deflects alpha particles.

⚛️ Subatomic Particles & Atomic Notation

• Electron mass: $9.11 \times 10^{-28},g$. Proton mass: $1.67 \times 10^{-24},g$.
• Atomic number $Z$: number of protons; identifies the element.
• Mass number $A$: total protons + neutrons. Number of neutrons = $A - Z$.
• Neutral atom: electrons = protons.

🔬 Isotopes & Atomic Mass

Isotopes: same $Z$ (protons) but different $A$ (neutrons). Example hydrogen isotopes: protium (1 proton, 0 neutrons), deuterium (1 p, 1 n), tritium (1 p, 2 n).

Atomic mass (amu): weighted average of naturally occurring isotopes: Atomic mass = sum(fraction_i × mass_i). Example: chlorine average $\approx 35.45,amu$ from percent abundances of $^{35}Cl$ and $^{37}Cl$.

🧭 Periodic Table Essentials

• Periodic law: element properties repeat periodically when arranged by increasing atomic number $Z$.
• Groups (vertical): elements in a group share chemical behavior (e.g., Group IA = alkali metals, Group VIIA = halogens).
• Periods (horizontal): energy levels.
• Representative (main group) elements = Groups IA–VIIIA. Transition elements = Group B (d-block). Inner transition = lanthanides & actinides.

⚙️ Metals, Nonmetals, Metalloids

• Metals: majority of elements; typically solids (except Hg), shiny, conductive, malleable, lose electrons to form cations.
• Nonmetals: vary in state, poor conductors, form anions by gaining electrons.
• Metalloids: intermediate properties; often semiconductors (e.g., Si).

🔋 Ionic Charge Trends & Predicting Ions

• Metals tend to lose electrons to achieve noble-gas configuration → form cations (+ charge). Group IA → $+1$, IIA → $+2$, IIIA → $+3$.
• Nonmetals tend to gain electrons → form anions (- charge). Group VA → $-3$, VIA → $-2$, VIIA → $-1$.
• Use group number to predict common ionic charge for main-group elements; transition metals require determining charge from compound stoichiometry or given name.

✅ Study Tips & Problem-Solving

• For atomic notation tasks: identify $Z$ and $A$, compute neutrons as $A - Z$.
• For isotope abundance problems: multiply masses by fractional abundances and sum.
• Memorize common ion charges, common polyatomic ions, and the periodic groups for rapid naming and formula writing.