A grain of salt, a drop of water, and a piece of copper look completely different, but each is made of tiny particles that are too small to see directly. Chemistry connects what you can observe at the human scale to a particle model made of atoms, which are the smallest units that still carry the identity of an element. In this course, you will repeatedly move between macroscopic evidence, a particle-level explanation, and a symbolic description like a formula.
To anchor that connection, examine familiar materials and then zoom to the level where their differences are stored in particle structure.
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Salt stays crystalline because its particles form a repeating pattern, while water flows because its particles move past one another, and copper conducts electricity because its electrons are mobile within a metal lattice. Those are different macroscopic properties with different particle causes, not just different appearances.
A substance has a uniform composition throughout. Two important kinds of substances are elements and compounds. An element contains only one type of atom, while a compound contains two or more types of atoms chemically bonded in a fixed ratio. A mixture contains two or more substances together without forming new bonds between all particles, so the proportions can vary.
Compare the particle sketches and examples to decide what is the same and what is different about these categories at the particle level.