6.3 Chemical Properties of and Changes in Matter
Key Takeaways
- An atom has a dense nucleus of protons (positive) and neutrons (neutral) surrounded by electrons in shells; the atomic number equals the proton count and defines the element.
- Elements are pure substances of one atom type, compounds combine two or more elements in fixed ratio, mixtures blend substances without fixed ratio, and solutions are homogeneous mixtures at the molecular level.
- An element's chemical properties are set mainly by its valence electrons; the periodic table groups elements with the same valence count, so vertical neighbors share reactivity patterns.
- Ionic, covalent, and metallic bonds differ in how electrons are shared or transferred, and the bond type determines a compound's conductivity, melting point, and solubility.
- Balanced chemical equations conserve mass: the same number of each atom appears on both sides, and reaction types — synthesis, decomposition, single displacement, double displacement, combustion — describe how atoms rearrange.
Subatomic particles, atomic number and mass number, isotopes, ions, and electron shells are developed in the previous section, "Atomic Structure, Isotopes, and Reading the Periodic Table."
Elements, Compounds, Mixtures, Solutions
| Category | Definition | Example | Homogeneous? |
|---|---|---|---|
| Element | Pure substance of one atom type | Gold (Au), oxygen (O₂) | Yes |
| Compound | Two or more elements chemically combined in fixed ratio | Water (H₂O), NaCl | Yes |
| Mixture | Two or more substances physically blended, variable ratio | Sand and salt; trail mix | No (heterogeneous) |
| Solution | Homogeneous mixture at the molecular level | Saltwater, air, brass | Yes |
A compound can be separated only by chemical means; a mixture can be separated by physical means (filtering, distilling, chromatography). Solutions have a solute (dissolved, e.g., salt) and a solvent (the dissolver, e.g., water).
Which of the following is a compound rather than an element or mixture?
Periodic Table Position and Chemical Properties
Chemical properties — reactivity, the type of bonds an element forms, whether it gains or loses electrons — are governed by valence electrons. The periodic table arranges elements so that members of a group (vertical column) share the same valence electron count.
- Group 1, alkali metals (Li, Na, K…): one valence electron, lose it easily, extremely reactive with water.
- Group 2, alkaline earth metals (Mg, Ca…): two valence electrons, less reactive than Group 1.
- Groups 3–12, transition metals: variable valence, form colored compounds.
- Group 17, halogens (F, Cl, Br…): seven valence electrons, gain one readily, very reactive nonmetals.
- Group 18, noble gases (He, Ne, Ar…): full valence shell, chemically inert.
Across a period, metallic character decreases and nonmetallic character increases. Down a group, metallic character increases and atomic radius grows. These trends let a 4-8 teacher predict, from position alone, that fluorine is more reactive than iodine and that sodium is more reactive than magnesium.
Chemical Bonds and Formulas
| Bond type | Mechanism | Typical properties | Example |
|---|---|---|---|
| Ionic | Transfer of electrons from metal to nonmetal; held by attraction of oppositely charged ions | High melting point, brittle, conducts when dissolved or molten | NaCl, MgO |
| Covalent | Sharing of electron pairs between nonmetals | Lower melting point, often poor conductors; polar vs nonpolar variants | H₂O, CO₂, CH₄ |
| Metallic | Pool of delocalized electrons shared across metal cations | Conductive, malleable, lustrous | Cu, Fe, Al |
A chemical formula uses element symbols and subscripts to show the atoms in one unit of a compound: H₂O means 2 hydrogen and 1 oxygen; Ca(OH)₂ means 1 Ca, 2 O, and 2 H. Coefficients in front of a formula tell how many units are involved: 2H₂O means two water molecules (4 H and 2 O atoms).
Balancing Chemical Equations
The law of conservation of mass requires the same number of each atom on both sides. To balance hydrogen combustion:
Unbalanced: H₂ + O₂ → H₂O
- Count atoms: H: 2 → 2; O: 2 → 1. Oxygen is unbalanced.
- Put a 2 before H₂O: H₂ + O₂ → 2H₂O. Now H: 2 → 4 (unbalanced), O: 2 → 2.
- Put a 2 before H₂: 2H₂ + O₂ → 2H₂O. Now H: 4 → 4, O: 2 → 2. Balanced.
The same approach — count, adjust one coefficient at a time, recount — works for any equation a 4-8 student will meet.
Reaction Types
- Synthesis (combination): A + B → AB. Example: 2H₂ + O₂ → 2H₂O.
- Decomposition: AB → A + B. Example: 2H₂O → 2H₂ + O₂ (electrolysis).
- Single displacement: A + BC → AC + B. Example: Zn + 2HCl → ZnCl₂ + H₂.
- Double displacement: AB + CD → AD + CB. Example: AgNO₃ + NaCl → AgCl + NaNO₃.
- Combustion: a hydrocarbon reacts with oxygen to produce CO₂ and H₂O, releasing heat. Example: CH₄ + 2O₂ → CO₂ + 2H₂O.
Real-Life and Cross-Disciplinary Reactions
The competency names six reactions a 4-8 teacher should connect to students' lives:
- Rusting (4Fe + 3O₂ → 2Fe₂O₃): slow oxidation of iron; salt water accelerates it by conducting ions.
- Burning fossil fuels (combustion of methane or octane): releases CO₂ and H₂O, drives engines, and adds greenhouse gases to the atmosphere.
- Photosynthesis (6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂): plants convert light energy into chemical bond energy.
- Cellular respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP): the reverse of photosynthesis, releasing stored energy.
- Chemical batteries: redox reactions in a cell push electrons through an external circuit; lead-acid car batteries and lithium-ion phone batteries both rely on electron transfer at electrodes.
- Digestion of food: enzymes catalyze hydrolysis reactions that break large molecules (proteins, starches, fats) into absorbable units (amino acids, glucose, fatty acids).
Applications Across the Sciences
Chemical properties drive materials science (alloys, semiconductors), biochemistry (enzyme shape and substrate fit), transportation (battery chemistry and combustion), medicine (drug-receptor interactions and MRI contrast agents), and telecommunications (doped silicon in chips and fiber-optic glass). For Texas 4-8 classrooms, the bridge from the periodic table to these applications is the same: an element's valence electrons set its bonds, its bonds set its properties, and its properties set the technologies it enables.
When the equation CH₄ + O₂ → CO₂ + H₂O is balanced, what is the coefficient on O₂?
Which type of chemical bond forms when a metal transfers electrons to a nonmetal?