1) those that are pure
2) those that are not pure, also known as mixtures.
These two groups are further broken into subgroups. Under pure substances, there are:
1) pure elements - substances (materials) in which all the atoms are identical. Most of these are monatomic, meaning they exist as a single atom. Some of the gases, oxygen, hydrogen, nitrogen, fluorine, & chlorine, are diatomic meaning they exist as a pair of bonded atoms. Elements cannot be broken down (decomposed) into other substances. and 2) pure compounds - substances (materials) in which atoms of different elements are bound together by bonds. Pure compounds are treated as though the individual atoms that make up the substance are just large "atoms" since this is how the substance behaves.Mixtures come in two types. The first type of mixtures are called homogeneous which means unable to be differentiated (visually separated) into its parts. In chemistry, these homogeneous mixtures are called solutions. Tea, coffee, brass, steel, and tap water are all examples of solutions, otherwise called homogeneous mixtures.
The second type of mixture is the heterogeneous. These mixtures are easily identified since the different materials that are in the mixture can be seen. An example of an heterogeneous mixture is salt & pepper sprinkled in one's hand. Another example is granite rocks like the rock material facing the front (west face) of Hadley Hall.
The property of mixtures that separates them from compounds is that the substances in mixtures can separated from each other by physical means such as distillation for salt water or filtering like is done in coffee.
Compounds can not be separated by physical means. A compound must undergo a chemical reaction in which the compound is decomposed. An example of the decomposition of a compound is the breakdown of sugar into carbon (C) and water (H2O). Eventhough a compound is a combination of elements, these elements are so tightly attached (bonded) to each other that they behave as a single unit.
Compounds can be further divided into two subgroups:
1) molecular compounds - those compounds in which only nonmetals occur. Molecular compounds can be gas, liquid or solid at room temperature. and 2) ionic compounds - those compounds which contain various combinations of metals and nonmetals. ALL ionic compounds will contain at least one metal and one nonmetal. ALL ionic compounds are solids at room temperature.The majority of materials found in the universe occur in the form of mixtures. In order to understand, lets assume for a moment that there are only 2 pure substances out there. From those two substances, we could produce thousands of different mixtures by putting into the mix different amounts of each pure substance. A mixture of 25% of compound 1 to 75% of compound 2 would be a different mixture than one in which 50% of compound 1 was mixed with 50% of compound 2. The more pure substances you have the more mixtures that can occur.
Very few pure elements have been found. Most of the elements in the Periodic Table occur ONLY in compounds. Those found in the pure state are some of the gases, O2, N2, H2, He, Ne, Xe, Ar, Kr, Rn, Cl2, C, Cu, Au, Pt and sometimes Ag. Of these, C, Cu, Pt, Au, and Ag are more often found in compounds or alloys with other metals. The pure forms of carbon are diamond and coal.