Cesium is a soft, silvery-gold element classified as an alkali metal, belonging to Group 1 of the periodic table. As one of the most reactive elements, it ignites spontaneously in air and reacts explosively with water, making it a challenging substance to handle. Density is a defining physical characteristic of cesium, relating to its unique atomic structure and phase behavior. The element is rare but has important applications, notably its use in the highly precise timekeeping of atomic clocks.
The Numerical Density of Solid Cesium
The density of cesium in its solid state provides a baseline measurement for this unique metal. At standard temperatures, the density of solid cesium is approximately \(1.879\) grams per cubic centimeter (\(g/cm^3\)), equivalent to \(1,879\) kilograms per cubic meter (\(kg/m^3\)). This relatively low density is noteworthy, especially considering cesium’s high atomic mass of \(132.9\) atomic mass units.
For comparison, this density is less than that of aluminum, a much lighter element on the periodic table. The measurement is typically taken at \(20\) degrees Celsius, below its melting point. While the precise density can vary slightly with temperature, the value consistently remains under \(2.0\) \(g/cm^3\). This numerical value results directly from how the cesium atoms pack together in their crystalline structure.
Density Change Due to Phase Transition
Cesium has one of the lowest melting points of any element, approximately \(28.4\) degrees Celsius (\(83.1\) degrees Fahrenheit). This low threshold means the element is often found in a liquid state in warmer environments or simply by the heat of a human hand. The density of liquid cesium near the melting point is about \(1.843\) \(g/cm^3\).
The transition from solid to liquid reveals a peculiar property: its density actually increases upon melting. Solid cesium has a slightly lower density than its liquid form near the melting point. This is unusual, as most substances contract and become denser as they solidify. Consequently, solid cesium would float on a pool of liquid cesium near the melting temperature.
Atomic Structure and Low Density
The surprisingly low density of cesium, despite its large atomic mass, is explained by its underlying atomic structure. Cesium is the largest stable atom on the periodic table, possessing a large atomic radius of about \(260\) picometers. This large size means that each atom occupies a disproportionately large volume relative to its mass.
As an alkali metal in Group 1, a cesium atom has only one valence electron to participate in metallic bonding. This single electron leads to very weak metallic bonds between the atoms. The atoms arrange themselves into a body-centered cubic (BCC) crystal structure, which is a relatively open and inefficient packing arrangement.
This combination of a massive but physically large atom, weak bonding, and inefficient packing results in a low mass-to-volume ratio. Low density is defined by this low mass-to-volume ratio.