Classify each of the following pure substances as an element or a compound: d. rust (Fe₂O₃)

Elements are pure substances that consist of only one type of atom and cannot be broken down into simpler substances by chemical means. Each element is defined by its atomic number, which represents the number of protons in its nucleus. Examples include hydrogen (H), oxygen (O), and iron (Fe).

Compounds are substances formed when two or more different elements chemically bond together in fixed proportions. They can be broken down into their constituent elements through chemical reactions. An example of a compound is water (H₂O), which consists of hydrogen and oxygen atoms.

A chemical formula is a symbolic representation of a compound that indicates the types and numbers of atoms present. For instance, rust is represented by the formula Fe₂O₃, which shows that it contains two iron (Fe) atoms and three oxygen (O) atoms. This formula helps in identifying the compound's composition and properties.