Aluminum foil is a staple in most households, used for wrapping leftovers and lining baking sheets. To classify this common material, we must use the basic organizational structure of matter used in chemistry. This system distinguishes between chemically pure substances and physical combinations of materials. Determining the nature of aluminum foil requires applying these definitions to its composition.
Defining Elements, Compounds, and Mixtures
Matter is categorized into elements, compounds, and mixtures based on the atoms they contain and how those atoms are joined.
An element is a pure substance consisting only of atoms with the same number of protons. Elements cannot be broken down into simpler substances by chemical means. Examples include gold, oxygen, and aluminum.
A compound is also a pure substance, formed when two or more different elements are chemically bonded in a fixed ratio. The resulting compound has properties entirely different from the individual elements, such as hydrogen and oxygen combining to form water (\(\text{H}_2\text{O}\)).
A mixture involves two or more substances that are simply combined physically, not chemically bonded, and can be in variable ratios. Since the components are not chemically united, they largely retain their individual properties and can often be separated by physical processes.
The Chemical Identity of Aluminum Foil
When applying these definitions, aluminum foil is primarily classified as an element. The vast majority of the material (typically 98.5% to 99.5% by weight) is the single element aluminum (\(\text{Al}\)) rolled into a thin sheet. This composition of essentially one type of atom is the definitive factor for its elemental classification.
Commercial foil is not perfectly pure, which introduces nuance. The small remaining percentage consists of trace amounts of other elements, such as iron and silicon, added to enhance strength and flexibility. Since these components are physically combined, the foil is technically a solid solution, or an alloy, which is a type of mixture.
A second feature is the ultra-thin, invisible layer of aluminum oxide (\(\text{Al}_2\text{O}_3\)) that forms on the surface when the metal is exposed to air. This surface layer is a compound because it consists of aluminum atoms chemically bonded with oxygen atoms in a fixed ratio. However, because this compound layer is only a few nanometers thick and the trace impurities are minor, the foil is classified based on its bulk composition: the elemental metal.
Aluminum: The Metal Element
The elemental nature of aluminum is confirmed by its place on the periodic table, where it is found with the symbol \(\text{Al}\) and the atomic number 13. This number signifies that every aluminum atom contains 13 protons in its nucleus, the defining characteristic of the element. Aluminum is a soft, lightweight, and highly ductile metal located in Group 13 and Period 3 of the table.
Aluminum’s ability to be rolled into the extremely thin sheets known as foil is directly linked to its malleability as a metal. The atoms are arranged in a metallic lattice structure, allowing them to slide past each other without breaking the material. The atomic structure means the atom has three valence electrons it readily gives up, forming the stable \(\text{Al}^{3+}\) ion in chemical reactions.
The thin, protective aluminum oxide layer is a product of this reactivity, as the metal readily reacts with oxygen in the air. This oxide layer, known as a passivation layer, is tough and non-porous. It immediately stops further oxidation and prevents the bulk metal from corroding, making elemental aluminum foil stable and durable for its many uses.