Cobalt (Co, atomic number 27) is a hard, lustrous, bluish-gray transition metal with strong magnetic properties, often grouped with nickel and iron. It is used in high-strength alloys and battery technology. Understanding its density is necessary for engineering and manufacturing. This discussion establishes the specific density value for solid cobalt and explains the factors that cause slight variations.
Understanding Density: A Quick Review
Density is a fundamental physical property that describes how much mass is contained within a specific volume. It quantifies the compactness of a substance and is calculated by dividing the total mass by the space it occupies.
The density of solids like cobalt is commonly expressed using two primary unit systems. In scientific settings, density is measured in grams per cubic centimeter (\(\text{g/cm}^3\)). The standard international (SI) unit, preferred in engineering, is kilograms per cubic meter (\(\text{kg/m}^3\)).
The Standard Density of Cobalt
The accepted standard density for pure, solid cobalt metal at room temperature is approximately \(8.90 \text{ g/cm}^3\). This measurement provides the mass-to-volume ratio under typical laboratory conditions. When converted to the SI unit system, this density value is \(8900 \text{ kg/m}^3\).
This density places cobalt among the heavier transition metals. Cobalt is slightly denser than iron (\(7.847 \text{ g/cm}^3\)) but is almost identical to its neighbor nickel (\(8.908 \text{ g/cm}^3\)).
The standard density is measured on macro-crystalline cobalt, the most common form found at ambient temperatures. Although \(8.90 \text{ g/cm}^3\) is widely cited, small variations exist in literature, sometimes ranging between \(8.86\) and \(8.92 \text{ g/cm}^3\), depending on purity and measurement technique.
Factors Influencing Cobalt’s Density
The actual density of a cobalt sample can fluctuate slightly due to specific physical conditions. One primary factor is temperature, which influences density through thermal expansion. As the temperature of a solid increases, the atoms vibrate more vigorously and move farther apart, causing the material’s volume to expand.
This volume increase, with mass remaining constant, results in a minor decrease in density. Temperature changes can also trigger polymorphism, a shift in cobalt’s internal atomic arrangement between two distinct crystal structures.
At room temperature, the stable form is the hexagonal close-packed (HCP) structure. As the temperature rises to approximately \(420^\circ\text{C}\), pure cobalt transitions to a face-centered cubic (FCC) structure. Since the FCC structure has a different packing efficiency than HCP, this change causes a measurable difference in the material’s density.