Overview
This lecture explains how chemical formulas represent the composition and structure of molecules, introduces empirical and molecular formulas, and discusses isomers.
Types of Chemical Formulas
- A molecular formula uses chemical symbols and subscripts to show the type and number of atoms in a molecule.
- A structural formula shows both the types and numbers of atoms plus how they are connected in the molecule.
- Ball-and-stick and space-filling models visually represent the arrangement and sizes of atoms in 3D.
Elements and Molecules
- Some elements exist as individual atoms while others form molecules of two or more atoms, like H₂, O₂, and N₂ (diatomic).
- Sulfur commonly exists as S₈, meaning eight atoms bonded together.
Subscripts and Coefficients
- A subscript (e.g., H₂) shows the number of atoms in a molecule; a coefficient (e.g., 2H) shows the number of separate atoms or molecules.
- H₂ = one molecule with two bonded hydrogen atoms; 2H = two separate hydrogen atoms; 2H₂ = two molecules of H₂.
Empirical vs. Molecular Formulas
- An empirical formula gives the simplest whole-number ratio of atoms in a compound.
- A molecular formula shows the actual number of atoms of each element in a molecule.
- Empirical and molecular formulas are related: the molecular formula is a whole-number multiple of the empirical formula.
Examples and Practice
- Glucose: Molecular formula is C₆H₁₂O₆; empirical formula is CH₂O.
- Metaldehyde: Molecular formula is C₈H₁₆O₄; empirical formula is C₂H₄O.
Isomers
- Isomers are compounds with the same molecular formula but different structures and chemical properties.
- Structural isomers differ in the atom-to-atom connections; spatial isomers have the same connectivity but differ in the 3D arrangement.
- Carvone exists in two spatial isomers with different smells (caraway vs. spearmint).
Key Terms & Definitions
- Molecular formula — actual number of each atom in a molecule.
- Empirical formula — simplest whole-number ratio of atoms in a compound.
- Structural formula — shows arrangement of atoms and bonds in a molecule.
- Diatomic molecule — molecule made of two atoms of the same element.
- Isomer — compounds with the same formula but different structures.
- Structural isomer — isomers with different atom-to-atom connections.
- Spatial isomer — isomers with different spatial arrangements of atoms.
Action Items / Next Steps
- Practice determining empirical and molecular formulas from atom counts.
- Watch the recommended video on isomers and spatial isomers via provided link.
- Review ball-and-stick and space-filling models for common molecules.