An element is a substance in which there is only one kind of atom. For example, Al and F₂ are both elements, but AlF₃ is a compound, not an element.

A compound is a substance in which the atoms of two or more different elements are bonded together in a specific ratio. A compound will have a unique set of physical and chemical properties (density, melting point, reactivity) that are quite different from those of its component elements. Because a compound has its own set of physical properties, it can not be separated by physical means (heating, sorting, magnetism, etc.)

The subscripts in a compound are numbers that indicate the ratio of each element in a compound. They are written just after the element they refer to. If no number is written then the subscript is understood to be one.

Example: Water is a compound formed from a 2:1 ratio of Hydrogen and Oxygen H₂O. It

behaves very differently from either Hydrogen or Oxygen.

A mixture is a combination of two or more compounds or elements. Unlike a compound, the atoms in a mixture are not bonded to each other, there is no fixed ratio. The properties of a mixture are not unique, but are a combination of the properties of the substances in the mixture. For this reason a mixture can be separated by physical means.

Examples: salt water (there is no fixed ratio of salt to water), air (a mixture of nitrogen, oxygen water, etc.)

A solution is a mixture in which the largest particle is an atom, ion, or molecule.