Overview
This lecture explains the structure and properties of ionic compounds, focusing on formulas, crystal lattice structures, and using Lewis dot diagrams to understand electron transfer and ion ratios.
Compounds and Ionic Compounds
- Compounds contain more than one type of element; if only one element, it is not a compound.
- Ionic compounds form when atoms exchange electrons, creating ions.
- Ionic compounds are typically solids with a crystal structure but can exist as liquids at high temperatures.
- The formula unit represents the simplest whole number ratio of ions in an ionic compound.
Formula Units and Ratios
- The chemical formula (e.g., NaCl) shows the lowest whole number ratio of positive to negative ions, not individual atoms.
- NaCl means for every sodium ion, there is one chloride ion, not just one of each atom.
- Ionic compounds consist of large numbers of ions in specific ratios forming a crystal lattice.
Crystal Lattice Structure and Properties
- Ions in ionic compounds are arranged in a three-dimensional, repeating crystal lattice.
- Each ion is surrounded by oppositely charged ions, contributing to the stability of the structure.
- High melting point: strong ionic bonds require much energy to separate ions.
- Hardness: difficult to scratch because ions are tightly held in place.
- Brittleness: if enough force shifts the ions, like charges align and repel, causing the crystal to break.
- Many ionic compounds are soluble in water, as water's partial charges attract ions.
- Conductivity: ionic compounds conduct electricity when melted or dissolved, but not as solids.
Lewis Dot Diagrams and Bond Formation
- Atoms gain or lose electrons to achieve stable, full outer electron shells (octet rule).
- Lewis dot diagrams help visualize electron transfer to form ions in ionic bonds.
- The ratio of ions formed depends on how many electrons each atom wants to lose or gain.
Example 1: Lithium Chloride (LiCl)
- Lithium loses one electron to chlorine; both become ions.
- The formula unit is LiCl (one lithium ion per chloride ion).
Example 2: Magnesium Chloride (MgCl₂)
- Magnesium loses two electrons; each chlorine gains one.
- Two chloride ions are needed for each magnesium ion.
Example 3: Aluminum Oxide (Al₂O₃)
- Each aluminum loses three electrons; each oxygen gains two.
- The ratio is two aluminum ions for every three oxide ions.
Key Terms & Definitions
- Ionic compound — A compound formed by the transfer of electrons, resulting in a lattice of positive and negative ions.
- Formula unit — The lowest whole number ratio of ions in an ionic compound.
- Crystal lattice — The repeating three-dimensional structure of ions in an ionic solid.
- Lewis dot diagram — A diagram showing valence electrons and electron transfer in atoms.
Action Items / Next Steps
- Practice drawing Lewis dot diagrams for other ionic compounds.
- Review key properties of ionic compounds for future assessments.