Simple Substances and Chemical Compounds
Classifying pure substances by the element types in their particles
Big idea: A simple substance contains only one chemical element, while a chemical compound contains two or more elements chemically joined in a fixed composition. The classification depends on element identity and particle structure, not on whether a name sounds simple or whether a sample looks uniform.
Learning goals:
- classify pure substances as simple substances or compounds from names, formulas, or particle models;
- connect macroscopic samples, submicroscopic particles, and chemical symbols;
- explain why O2 is simple but H2O and NaCl are compounds;
- distinguish a compound from a mixture;
- read element ratios in introductory formulas;
- recognize the limits of molecular, ball-and-stick, and lattice models.
1. Start with pure substances
A pure substance has a definite composition and characteristic properties. At this level, pure substances are divided into two broad classes. A simple substance (also called an elemental substance in some textbooks) is made only from atoms of one chemical element. A chemical compound is a pure substance whose particles or extended structure contain atoms or ions of at least two different elements in a definite chemical arrangement.