Gallium (Ga), atomic number 31, is a soft, silvery-blue metal classified as a poor metal belonging to the boron group. One of its most remarkable characteristics is its unusually low melting point of approximately \(29.76\) degrees Celsius, meaning it can melt when held in a person’s hand. This allows it to exist as a liquid near room temperature, a property shared by only a few other elements, such as mercury and cesium. Understanding gallium’s density is key to explaining its unusual behavior.
Defining Gallium’s Specific Density
Density is a fundamental physical property that quantifies the mass contained within a specific volume, typically expressed in \(\text{g/cm}^3\). Solid gallium, measured at \(25\) degrees Celsius, has a density of approximately \(5.904 \text{ g/cm}^3\). This value is comparable to other metals, though less dense than copper (\(8.96 \text{ g/cm}^3\)).
The density changes when gallium transitions into its liquid state. Liquid gallium, measured at its melting point of \(29.76\) degrees Celsius, has a density of approximately \(6.095 \text{ g/cm}^3\). Comparing these figures reveals a highly unusual phenomenon: the liquid form is denser than its solid form, contrary to nearly all other substances.
The slight increase in density from the solid to the liquid state is a difference of about \(3\) percent, which has significant implications for how the element behaves. This precise measurement highlights gallium’s status as an outlier in the world of metals.
The Unique Density Anomaly
The fact that liquid gallium is denser than its solid form is known as the density anomaly. Most materials contract when they freeze, making the solid state denser than the liquid state. Gallium, however, expands upon freezing, similar to water, which also exhibits this rare behavior. This expansion results from the unique arrangement of gallium atoms in their solid, crystalline structure, driven by the directional covalent character of its metallic bonding.
In the solid state, gallium atoms do not pack in a simple, efficient lattice. Instead, they form distinct pairs, referred to as \(\text{Ga}_2\) dimers, which arrange themselves in an open crystal lattice structure that is less tightly packed than the liquid state. When solid gallium melts, the energy breaks the rigid, open lattice, allowing the atom pairs to collapse closer together. This transition results in a more compact arrangement of atoms, leading to increased density. This structural shift is also indicated by a negative slope in gallium’s melting curve on its phase diagram.
Practical Applications of Gallium’s Properties
The combination of gallium’s low melting point, high boiling point, and density anomaly makes it valuable in modern technology. Its low melting point allows it to be used in non-toxic thermometers as a safer replacement for mercury. Gallium is also mixed with metals like indium and tin to create low-melting alloys, such as Galinstan, which are liquid at room temperature for electronic and medical applications.
The most widespread use of gallium is in semiconductor technology, forming compounds with elements like arsenic and nitrogen. Gallium Arsenide (\(\text{GaAs}\)) and Gallium Nitride (\(\text{GaN}\)) have electronic properties superior to traditional silicon in certain applications. \(\text{GaAs}\) is used to manufacture high-speed integrated circuits for mobile phones, while \(\text{GaN}\) is used in the production of energy-efficient blue and white Light Emitting Diodes (LEDs) and power transistors.
Liquid gallium and its alloys are also used in thermal management due to their high thermal conductivity. They function as a liquid metal thermal interface material to efficiently transport heat away from hot semiconductor chips in high-performance computing. This utility is linked to gallium’s ability to remain liquid over an extremely wide temperature range, extending up to \(2,204\) degrees Celsius.