The combination of aluminum and oxygen forms aluminum oxide, which has the chemical formula \(\text{Al}_2\text{O}_3\). This material is an ionic compound, formed by the electrostatic attraction between positively and negatively charged ions. This bonding is typical when a metal like aluminum bonds with a non-metal like oxygen. Understanding this specific ratio requires examining the electrical charges the individual elements typically form.
Oxidation States of the Elements
Aluminum is a metal found in Group 13 of the periodic table. It loses its three valence electrons to achieve a stable configuration, resulting in a positively charged ion, or cation, represented as \(\text{Al}^{3+}\). This +3 oxidation state is the most common state for aluminum in its compounds.
Oxygen is a non-metal located in Group 16. To reach a stable configuration, oxygen accepts two electrons, resulting in a negatively charged ion, or anion, represented as \(\text{O}^{2-}\). The -2 charge is the standard oxidation state for oxygen in most compounds. When these two elements combine, the total positive charge from the aluminum ions must balance the total negative charge from the oxygen ions to form a neutral compound.
The Crisscross Method for Ionic Compounds
The formula for aluminum oxide is derived by ensuring the net charge of the compound is zero, a process visualized using the crisscross method. This technique uses the magnitude of the charge on one ion to determine the subscript of the other element in the formula. Starting with the ions \(\text{Al}^{3+}\) and \(\text{O}^{2-}\), the numerical value of the aluminum charge (3) becomes the subscript for oxygen, and the oxygen charge (2) becomes the subscript for aluminum.
This exchange results in the formula \(\text{Al}_2\text{O}_3\), which is the chemical formula for aluminum oxide. This ratio of two aluminum atoms to three oxygen atoms ensures electrical neutrality. Two \(\text{Al}^{3+}\) ions contribute a total positive charge of \(+6\), while three \(\text{O}^{2-}\) ions contribute a total negative charge of \(-6\). This perfectly cancels out to a net charge of zero, guaranteeing the lowest whole-number ratio of ions needed to form the neutral ionic compound.
Uses of the Final Compound
The resulting compound, aluminum oxide, is commonly known as alumina. It is a highly versatile ceramic material with a wide range of applications. Its exceptional hardness, rated at 9 on the Mohs scale, makes it popular for use as an abrasive in sandpaper, grinding wheels, and blasting media.
Alumina also possesses a high melting point and is chemically inert, making it ideal for use in refractory materials designed to withstand high temperatures in furnaces and kilns. In nature, aluminum oxide is found as the mineral corundum. The presence of trace impurities, such as chromium, in corundum creates the red color of rubies, while other impurities lead to the various colors of sapphires.