⚗️ Chemistry Study Guide
- Atoms & molecules — chemistry begins at the atomic level
- Basic arithmetic — needed for mole calculations
- Describe atomic structure and explain the role of protons, neutrons, and electrons
- Use the periodic table to predict element properties and bonding behavior
- Distinguish between ionic and covalent bonding and predict bond type
- Balance chemical equations and calculate molar quantities using stoichiometry
- Explain acids, bases, and pH and predict the result of neutralization reactions
Chemistry is the game of rearranging atoms. In any chemical reaction, atoms do not appear or disappear, they just bond differently (conservation of mass). When you read a chemical formula, ask: which atoms, and how many of each, are bonded together?
Chemistry is the science of matter and its transformations. Why does aspirin relieve pain? Why does iron rust but gold doesn't? How does a battery store electricity? All of these questions have chemical answers. Acid-base chemistry directly connects to blood pH regulation in biology, and oxidation-reduction reactions are the basis of fuel cells and batteries. Chemistry is the bridge between physics (atoms) and biology (life), and without it, modern medicine, materials science, and energy technology would not exist.
- Atomic structure: protons (atomic #) + neutrons (mass #) + electrons. Same group = similar properties
- Bonding: ionic (metal + nonmetal, electron transfer), covalent (nonmetal + nonmetal, electron sharing)
- Reactions: balanced equations — atoms conserved. Types: synthesis, decomposition, single/double displacement
- Acid-base: acids donate H⁺, bases accept H⁺. pH 7 = neutral; <7 acidic; >7 basic
- Redox: oxidation = lose electrons; reduction = gain electrons. OIL RIG
Antoine Lavoisier (1789) proved the law of conservation of mass by carefully weighing sealed combustion reactions, overthrowing phlogiston theory — chemistry's first paradigm shift. Nearly a century later, Dmitri Mendeleev published the periodic table in 1869, famously leaving gaps for elements yet to be discovered. When gallium (1875) and germanium (1886) were found matching his predictions exactly, the periodic table became one of science's greatest predictive triumphs.
1. Atomic Structure
All matter is made of atoms. An atom consists of a nucleus (containing protons and neutrons) surrounded by electrons.
| Particle | Charge | Mass (amu) | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | ≈0 | Electron cloud (orbitals) |
- Atomic number (Z): number of protons — defines the element
- Mass number (A): protons + neutrons
- Isotopes: same element (same Z), different number of neutrons
- Ion: atom that has gained or lost electrons (anion = negative, cation = positive)
Electron Configuration
Electrons occupy energy levels (shells) and subshells (s, p, d, f). Fill lowest energy first (Aufbau principle).
Oxygen (Z = 8): 1s² 2s² 2p⁴
Sodium (Z = 11): 1s² 2s² 2p⁶ 3s¹
2. The Periodic Table
Elements are arranged by increasing atomic number. Elements in the same column (group/family) share similar properties.
- Group 1 — Alkali Metals: Li, Na, K… Very reactive, 1 valence electron
- Group 2 — Alkaline Earth Metals: Mg, Ca… 2 valence electrons
- Group 17 — Halogens: F, Cl, Br… Very reactive nonmetals, 7 valence electrons
- Group 18 — Noble Gases: He, Ne, Ar… Inert (full outer shell)
- Transition Metals: Groups 3–12; include Fe, Cu, Au, Ag
- Atomic radius: decreases across a period (left → right); increases down a group
- Electronegativity: increases across a period; decreases down a group (F is highest)
- Ionization energy: energy to remove an electron; increases across a period
3. Chemical Bonding
- Ionic bond: transfer of electrons between a metal and nonmetal. Forms a crystal lattice. Example: NaCl
- Covalent bond: sharing of electrons between nonmetals. Can be single (1 pair), double (2 pairs), or triple (3 pairs). Example: H₂O, CO₂
- Polar covalent: unequal sharing due to electronegativity difference. Example: H₂O (oxygen is more electronegative)
- Metallic bond: electrons are delocalized in a "sea" — explains conductivity and malleability of metals
- Valence electrons: electrons in the outermost shell; determine bonding behavior
4. Chemical Reactions
A chemical reaction rearranges atoms to form new substances. The law of conservation of mass requires that equations be balanced.
Types of Reactions AP Exam
- Synthesis: A + B → AB (two substances combine)
- Decomposition: AB → A + B (one substance breaks down)
- Single replacement: A + BC → AC + B
- Double replacement: AB + CD → AD + CB
- Combustion: hydrocarbon + O₂ → CO₂ + H₂O (e.g., CH₄ + 2O₂ → CO₂ + 2H₂O)
Balancing Equations
Count: H: 2 vs 2 ✓, O: 2 vs 1 ✗
Balanced: 2H₂ + O₂ → 2H₂O
Count: H: 4 vs 4 ✓, O: 2 vs 2 ✓
5. Acids, Bases, and pH
- Acid: donates H⁺ ions (proton donor). pH < 7. Examples: HCl, H₂SO₄, vinegar (acetic acid)
- Base: accepts H⁺ ions (proton acceptor). pH > 7. Examples: NaOH, NH₃, baking soda
- Neutral: pH = 7 (pure water)
| pH Range | Classification | Example |
|---|---|---|
| 0–2 | Strong acid | HCl (stomach acid: ~2) |
| 3–6 | Weak acid | Vinegar (~3), coffee (~5) |
| 7 | Neutral | Pure water |
| 8–11 | Weak base | Baking soda (~9), soap (~10) |
| 12–14 | Strong base | NaOH (bleach: ~13) |
Neutralization AP Exam
HCl + NaOH → NaCl + H₂O
6. Stoichiometry
Stoichiometry uses balanced chemical equations to calculate the amounts of reactants and products in a reaction. The key unit is the mole.
- 1 mole = 6.022 × 10²³ particles (Avogadro's number)
- Molar mass = mass of 1 mole in grams = atomic/molecular mass in g/mol
Example: H₂O has molar mass = 2(1) + 16 = 18 g/mol - Mole conversions:
grams ÷ molar mass = moles
moles × 6.022×10²³ = number of particles
The coefficients in a balanced equation give the mole ratios of reactants and products.
This means: 1 mol N₂ reacts with 3 mol H₂ to produce 2 mol NH₃
How many grams of NH₃ are produced when 28 g of N₂ reacts completely with excess H₂?
Step 2: Mole ratio (N₂ : NH₃ = 1 : 2) → 1 mol N₂ produces 2 mol NH₃
Step 3: Molar mass of NH₃ = 14 + 3(1) = 17 g/mol
Step 4: Mass = 2 mol × 17 g/mol = 34 g NH₃
The limiting reagent is the reactant that runs out first and determines the maximum amount of product formed.
- To find the limiting reagent: convert both reactants to moles, then use mole ratios to determine which one produces the least product.
- The excess reagent is left over after the reaction is complete.
8. Practice Problems
- An element has atomic number 17 and mass number 35. How many protons, neutrons, and electrons does it have?
- Write the electron configuration for Calcium (Z = 20).
- Classify the bond in MgCl₂: ionic or covalent? Explain.
- Balance: Fe + O₂ → Fe₂O₃
- A solution has [H⁺] = 0.001 M. What is its pH? Is it acidic or basic?
- How many grams of water are produced when 4 mol of H₂ reacts with excess O₂? (Balanced: 2H₂ + O₂ → 2H₂O)
- Protons = 17, Neutrons = 35−17 = 18, Electrons = 17 (neutral atom)
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
- Ionic — Mg is a metal, Cl is a nonmetal; Mg transfers electrons to Cl
- 4Fe + 3O₂ → 2Fe₂O₃
- pH = −log(0.001) = −log(10⁻³) = 3 → Acidic
- Mole ratio (H₂ : H₂O = 2 : 2 = 1 : 1) → 4 mol H₂ produces 4 mol H₂O
Molar mass of H₂O = 18 g/mol → 4 × 18 = 72 g of water
How do atoms and molecules combine to form the machinery of living cells?
Proteins, DNA, and lipids are all carbon-based organic compounds. The covalent and ionic bonds you just studied are precisely what hold the molecules of life together — chemistry is the foundation of biology.
BiologyChemistry is the common language of life science, medicine, and materials engineering. Understanding the periodic table and reaction equations now opens the door to biochemistry, pharmacology, and environmental science.
- Balance chemical equations: atoms on both sides must be equal
- 1 mol = 6.02×10²³ particles (Avogadro's number)
- pH scale: 7 = neutral, <7 = acidic, >7 = basic
- Oxidation/reduction always occur together (redox pair)
Review this material at increasing intervals to commit it to long-term memory.