Overview
This lecture explains how to classify substances as ionic or covalent compounds, their properties, examples, and how their bonds and structures determine their behavior.
Types of Compounds
- Compounds are chemical combinations of two or more elements.
- There are two main types: ionic compounds and covalent compounds.
- Ionic compounds form from metals and non-metals; covalent compounds form from two non-metals or a metalloid and a non-metal.
Identifying Elements on the Periodic Table
- Metals are mostly on the left (except hydrogen); non-metals are on the right; metalloids form a stair-step pattern in between.
- Use periodic table positions to distinguish metals, non-metals, and metalloids.
Ionic Compounds and Bonds
- Ionic compounds are formed by the transfer of electrons between metal and non-metal atoms.
- Metals lose electrons to become positively charged cations; non-metals gain electrons to become negatively charged anions.
- Ionic bonds are electrostatic attractions between cations and anions.
- Common examples: sodium chloride (salt), sodium bicarbonate (baking soda), sodium hydroxide (soap).
- Ionic compounds have high melting and boiling points, are hard but brittle, and form crystal lattice structures.
- Conduct electricity in aqueous or molten state (as electrolytes), but not when solid.
Covalent Compounds and Bonds
- Covalent compounds result from sharing electrons between non-metallic elements, forming molecules.
- Covalent bonds are strong, but forces between molecules are weak, leading to low melting and boiling points.
- Examples: sucrose (sugar), water, carbon dioxide.
- Covalent compounds are often soft, flexible, more flammable, and can be gases, liquids, or soft solids at room temperature.
- Generally do not conduct electricity when dissolved in water.
Polarity and Electronegativity
- Polarity depends on electronegativity difference between bonded atoms.
- Polar covalent bonds have unequal electron sharing (partial charges).
- Nonpolar covalent bonds have equal sharing (no charge separation).
- If electronegativity difference is ≥ 2.0, bond is ionic.
Summary Table of Properties
- Ionic: high melting/boiling points, hard, brittle, conduct electricity in aqueous form, form crystals.
- Covalent: low melting/boiling points, soft, flexible, flammable, do not conduct electricity, can be polar or nonpolar.
Key Terms & Definitions
- Ionic bond — Attraction between oppositely charged ions (cations and anions) formed by electron transfer.
- Covalent bond — Attraction formed by sharing electrons between atoms.
- Cation — Positively charged ion, usually formed from metals.
- Anion — Negatively charged ion, usually formed from non-metals.
- Electronegativity — An atom's ability to attract shared electrons in a bond.
- Electrolyte — Substance that conducts electricity when dissolved in water due to presence of ions.
Action Items / Next Steps
- Review periodic table for element classification.
- Prepare for next topic: formation of ions.