Overview
This lecture explains how atoms combine to form molecules, compounds, and mixtures, and covers chemical formulas and the differences between these terms.
Molecules vs. Compounds
- A molecule is two or more atoms held together by chemical bonds.
- Molecules can be made of the same element (e.g., O₂, N₂) or different elements (e.g., H₂O, CO₂).
- A compound contains two or more different elements chemically bonded together.
- All compounds are molecules, but not all molecules are compounds.
- Elements in a compound are always present in fixed proportions.
Chemical Formulas and Notation
- Chemical formulas use symbols and subscripts to show the number of each atom (e.g., H₂O means two hydrogens, one oxygen).
- No subscript means only one atom of that element (e.g., CO₂ means one carbon, two oxygens).
- Brackets in formulas group atoms; a subscript outside the bracket multiplies everything inside (e.g., Ca(OH)₂).
- Complex compounds, like H₂SO₄ (sulfuric acid), contain multiple different atoms.
Structure of Compounds
- Some compounds, like sodium chloride (NaCl), do not form small molecules but large ionic structures.
- Formulas for ionic compounds represent the ratio of ions, not distinct molecules (e.g., 1:1 ratio in NaCl).
- Ionic compounds form giant lattice structures with millions or billions of atoms.
Mixtures
- A mixture contains two or more substances physically mixed, not chemically bonded.
- Substances in a mixture retain their individual properties and can be separated physically.
- Examples include combining oxygen molecules, sodium chloride, helium atoms, and carbon dioxide.
Key Terms & Definitions
- Molecule — A group of two or more atoms bonded together.
- Compound — A substance containing two or more different elements bonded together.
- Chemical Formula — A shorthand way to show which atoms and how many of each are in a molecule or compound.
- Mixture — A combination of different substances not chemically bonded and easily separated.
- Ionic Bond — A chemical bond formed by the transfer of electrons, resulting in large lattice structures.
Action Items / Next Steps
- Review chemical formula notation and practice interpreting them.
- Learn separation techniques for mixtures: filtration, crystallization, distillation.
- Look up symbols and names of elements on the periodic table as needed.