Atoms are the units that make up elements, or pure substances. Atoms are extremely tiny—it would take millions of atoms just to cover up the dot of an “i” on this page.
Atomic theory, in a very simplified summary, states that everything is made up of atoms and that there is a fixed number of different types of atoms. Different atoms make up different elements. This means that all of the atoms of a particular element, like gold or oxygen, are of the same type.
Atoms combine to make elements, and different elements combine and react with each other to form gases, liquids, and solids.