Matter, Energy, and Atoms
Chapters 3 and 4: review the ideas, then test yourself.
Chapter 3: Matter and Its Properties
States of matter
| Shape | Volume | Particles | |
|---|---|---|---|
| Solid | Definite | Definite | Tightly packed, fixed positions, vibrate |
| Liquid | Takes container's shape | Definite | Close together, slide past each other |
| Gas | Takes container's shape | Fills container | Far apart, move freely; compressible |
Phase changes and energy
| Change | Name | Energy |
|---|---|---|
| Solid → liquid | Melting | Endothermic (absorbs) |
| Liquid → gas | Vaporization (boiling, evaporation) | Endothermic |
| Solid → gas | Sublimation | Endothermic |
| Liquid → solid | Freezing | Exothermic (releases) |
| Gas → liquid | Condensation | Exothermic |
| Gas → solid | Deposition | Exothermic |
Why: energy is needed to pull particles apart (overcome attractions), so moving toward gas is endothermic. Particles moving closer together release energy, so moving toward solid is exothermic.
Element, compound, or mixture
- Element: one kind of atom (Fe, O₂). Cannot be broken down chemically.
- Compound: two or more elements chemically bonded in a fixed ratio (H₂O, NaCl). Can be broken down only by chemical changes.
- Mixture: two or more substances physically combined, variable composition, separable by physical means.
- Homogeneous mixture (solution): uniform throughout (saltwater, air). Heterogeneous: visibly different parts (soil, salad, granite).
Properties and changes
- Physical property: observed without changing identity (color, melting point, density, hardness).
- Chemical property: describes ability to change into new substances (flammability, reactivity with acid).
- Physical change: same substance, new form (melting, dissolving, tearing). Chemical change: new substance forms. Clues: gas bubbles, color change, precipitate, light or heat, odor.
Conservation of mass
Mass is neither created nor destroyed in a chemical reaction: mass of reactants = mass of products. Example: 10.0 g reactant that gives off 3.0 g of gas leaves 7.0 g of solid.
Chapter 4: The Atom
Subatomic particles
| Particle | Charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | ≈0 (1/1836) | Electron cloud, outside nucleus |
Scientists and the atom
| Scientist | Contribution |
|---|---|
| Democritus | Matter is made of tiny indivisible particles: atomos. An idea, not an experiment. |
| Dalton | Atomic theory: elements are made of atoms; atoms of an element are identical; compounds form from fixed ratios of atoms; reactions rearrange atoms. Modern update: atoms are divisible (subatomic particles) and isotopes of an element differ in mass. |
| Thomson | Cathode ray tube: found negative particles (electrons). Plum pudding model: electrons embedded in a positive sphere. |
| Millikan | Oil drop experiment: measured the electron's charge, which allowed its mass to be calculated. |
| Rutherford | Gold foil experiment: most alpha particles passed through, a few bounced back. Atom is mostly empty space with a small, dense, positive nucleus. |
| Bohr | Quantum atom: electrons orbit the nucleus in fixed energy levels. |
| Chadwick | Cloud chamber work: discovered the neutron (neutral, mass ≈ proton). |
Protons, neutrons, and electrons
- Atomic number (Z) = # protons. Neutral atom: # electrons = # protons.
- Mass number (A) = # protons + # neutrons, so neutrons = A − Z.
- Cation (+): lost electrons. Anion (−): gained electrons. Electrons = Z − charge.
- Ions change electrons only. Isotopes change neutrons only.
- Example: Mg²⁺ has 12 protons, 12 neutrons (Mg-24), and 10 electrons.
Isotope notation and average atomic mass
- Notation: mass number on top left, atomic number bottom left, symbol, charge top right. Also written carbon-14 or C-14.
- On the periodic table: whole number = atomic number; decimal = average atomic mass (not the mass number).
- Average atomic mass = Σ (isotope mass × fractional abundance). Convert % to decimal first (69.15% → 0.6915).
- Example: Cu-63 (62.93 amu, 69.15%) and Cu-65 (64.93 amu, 30.85%): 0.6915 × 62.93 + 0.3085 × 64.93 = 63.55 amu.