Chromium (III) oxide is an inorganic compound widely recognized for its intense color and stability, making it a popular pigment in various industries. This compound, often called chromia, forms a crystalline solid with a high degree of chemical inertness. Determining its accurate formula requires examining the charges carried by its constituent ions.
Identifying the Correct Chemical Formula
The correct chemical formula for Chromium (III) Oxide is \(\text{Cr}_2\text{O}_3\). This formula represents the simplest whole-number ratio of the elements chromium (\(\text{Cr}\)) and oxygen (\(\text{O}\)) required to form a stable, electrically neutral compound. \(\text{Cr}_2\text{O}_3\) is an empirical formula, showing that for every two chromium atoms, there are three oxygen atoms present in the compound’s structure.
The formula is derived from the need for the positive and negative charges of the ions to perfectly cancel each other out. This process of charge balancing is a fundamental requirement for the existence of any stable ionic compound. The subscripts of 2 and 3 in the formula indicate the necessary proportions of each element.
Understanding the Ionic Components
The name Chromium (III) Oxide follows the standard nomenclature for ionic compounds containing a transition metal, which can exhibit multiple oxidation states. The Roman numeral “III” specifically indicates the charge, or oxidation state, of the chromium ion (\(\text{Cr}\)). Therefore, the chromium ion in this compound is a cation with a positive three charge, represented as \(\text{Cr}^{3+}\).
The second part of the name, “oxide,” refers to the anion derived from oxygen. Oxygen is a member of Group 16 on the periodic table and typically forms an ion with a negative two charge, represented as \(\text{O}^{2-}\). The compound is formed through the strong electrostatic attraction between the positively charged chromium ions and the negatively charged oxide ions.
Balancing the Charges to Derive the Formula
An ionic compound must always be electrically neutral, meaning the total positive charge from the cations must precisely equal the total negative charge from the anions. To achieve this neutrality in Chromium (III) Oxide, the \(\text{Cr}^{3+}\) (charge \(+3\)) and \(\text{O}^{2-}\) (charge \(-2\)) ions must combine in a specific ratio.
To find the lowest common multiple (LCM) of the charges, which is 6, one must determine the smallest number of each ion that results in a net charge of zero. Two chromium ions (\(2 \times +3 = +6\)) provide a total positive charge that balances three oxide ions (\(3 \times -2 = -6\)). Since the charges cancel each other out, the compound is electrically neutral, and the resulting formula is \(\text{Cr}_2\text{O}_3\).
Notable Physical Properties and Uses
Chromium (III) Oxide is a hard, brittle material that naturally occurs as the rare mineral eskolaite. It exhibits a characteristic deep green color, often known as viridian, and is highly insoluble in water, alcohol, and acetone. The compound has a very high melting point of approximately 2,435 degrees Celsius, making it an extremely thermally stable substance.
This stability and color lead to its widespread use as a durable green pigment in applications like paints, ceramics, and glass. Due to its hardness, the compound is also used as an abrasive for polishing metal and optical surfaces. Furthermore, its heat-resistant properties make it a component in refractory materials used to line furnaces and kilns.