Overview
This lecture covers the basics of molecules and compounds, including chemical formulas, differences between compounds and mixtures, types of chemical formulas, and classification of molecular and ionic compounds.
Elements, Compounds, and Mixtures
- Sodium (Na) and chlorine (Cl) are reactive elements that form table salt (NaCl), which has different properties than the original elements.
- Compounds are substances with two or more elements chemically bonded in fixed ratios.
- Mixtures contain two or more substances not chemically bonded and can vary in composition.
- Properties of compounds differ from their constituent elements.
Chemical Formulas and Ratios
- Compounds have constant, definite proportions; e.g., H₂O always has two hydrogens and one oxygen.
- The law of constant composition states all samples of a compound have the same element ratios.
- Mass ratios can be calculated by dividing the mass of one element by another within a compound.
Types and Writing of Chemical Formulas
- Chemical formulas (like H₂O, CO₂) show elements present and their amounts; subscripts of one are omitted.
- The order of elements in formulas lists the more metallic element first (leftmost on the periodic table).
- Polyatomic ions (many atoms bonded, carrying a charge) are written as a group, e.g., NO₃⁻ for nitrate.
Empirical, Molecular, and Structural Formulas
- Empirical formula: simplest whole-number ratio of elements (e.g., CH₂O for glucose).
- Molecular formula: actual number of atoms in a molecule (e.g., C₆H₁₂O₆).
- Structural formula: shows how atoms are connected (lines represent bonds; each line is two shared electrons).
Molecular Models
- Ball-and-stick models represent atoms as colored spheres and bonds as sticks.
- Space-filling models represent overlapping electron clouds.
- Common color codes: hydrogen (white), carbon (black), oxygen (red), nitrogen (blue).
Classification of Substances
- Pure substances: only one kind of particle; classified as elements or compounds.
- Elements can be atomic (single atoms, e.g., Ne) or molecular (bonded atoms, e.g., O₂).
- Compounds: two or more elements bonded; can be molecular (nonmetals only) or ionic (metal + nonmetal or polyatomic ions).
Molecular vs. Ionic Compounds
- Molecular compounds consist of nonmetal atoms bonded in molecules (e.g., CO₂).
- Ionic compounds consist of cations (positively charged) and anions (negatively charged) arranged in a crystal lattice (e.g., NaCl).
Writing Ionic Formulas and Charges
- Cations are written first, then anions; charges must balance to create a neutral compound.
- Group 1 elements: +1 charge; Group 2: +2; Group 17: -1; Group 16: -2; Group 15: -3.
- Use subscripts to balance charges; simplify ratios to lowest terms.
- Polyatomic ions must be grouped in parentheses if more than one is present.
Key Terms & Definitions
- Compound — a substance with two or more elements chemically bonded in a fixed ratio.
- Mixture — a combination of substances not chemically bonded, variable composition.
- Law of Constant Composition — all samples of a compound have the same element ratios.
- Empirical Formula — simplest whole-number ratio of elements.
- Molecular Formula — exact number of each type of atom in a molecule.
- Structural Formula — shows atom connectivity and bonds.
- Molecular Compound — composed of nonmetals bonded together.
- Ionic Compound — composed of cations and anions held in a lattice.
- Cation — positively charged ion.
- Anion — negatively charged ion.
- Polyatomic Ion — an ion made of multiple atoms bonded, acting as a unit.
Action Items / Next Steps
- Memorize diatomic elements (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂).
- Review polyatomic ions list provided on Blackboard.
- Practice calculating mass ratios and writing formulas for compounds.
- Complete assigned homework problems using summary process provided.