Gallium is a soft, silvery metal, represented by the symbol Ga, that challenges the perception of what metal should be. It has a remarkably low melting point of 29.76 degrees Celsius (85.57 degrees Fahrenheit). This means a solid piece of gallium will liquefy in the palm of your hand, mimicking the texture of a dense fluid. This ability to transition from a solid to a liquid so close to room temperature has captured the public’s imagination.
The Science Behind Gallium’s Softness
The unusually low melting point of gallium is a consequence of its distinctive atomic arrangement in the solid state. Unlike most metals, which form simple, tightly packed crystal structures, solid alpha-gallium (alpha-Ga) adopts a complex orthorhombic structure. This unique arrangement features pairs of gallium atoms that bond tightly together, forming Ga2 dimers.
The bonds within these atomic pairs are covalent, a stronger type of bond typically found in non-metals. However, the bonds between these Ga2 dimers are significantly weaker and more typical of metallic bonding. This weak force requires little energy to overcome, causing the crystal lattice to destabilize and melt just above room temperature.
This structural oddity causes liquid gallium to be denser than its solid form, a characteristic shared with water. As liquid gallium solidifies, it expands by approximately 3.1 percent, which is the opposite behavior of most metals. This expansion occurs because the liquid atoms transition into the less-efficiently packed solid dimer structure.
Addressing Toxicity and Safety Concerns
Elemental gallium has a low toxicity profile, making it much safer to handle than other liquid metals like mercury. It has no known natural biological role and is not readily absorbed through the skin or the digestive tract due to its insolubility in water. If small amounts of the metal are accidentally ingested, they pass through the body without being absorbed.
The main safety concern with gallium is not human health but its destructive effect on other materials. Gallium is highly corrosive to certain metals, most notably aluminum. When it contacts aluminum alloys, gallium readily diffuses into the grain boundaries, causing the aluminum structure to become extremely brittle.
This corrosive action means gallium should never be handled near aluminum items, such as common tools, jewelry, or electronics casings. Due to its expansion upon freezing, liquid gallium should not be stored in rigid, sealed containers. If it solidifies inside a sealed container, the volume expansion could cause the vessel to rupture.
Everyday Uses of Gallium
Beyond its novelty as a low-melting metal, gallium is an important element in modern technology, particularly in the electronics industry. Its most common applications involve compounds like gallium arsenide (GaAs) and gallium nitride (GaN), which are semiconductors. Gallium arsenide is widely used in high-speed and microwave circuits necessary for wireless communication and satellite technology.
Gallium nitride is the foundational material for blue and violet light-emitting diodes (LEDs), which revolutionized lighting and display technology, including Blu-ray players. These compounds are also preferred over silicon in specialized applications, such as solar cells used on space vehicles and satellites, because of their superior efficiency in converting light into electricity.
The element’s low melting point is exploited in certain alloys, such as Galinstan, a mixture of gallium, indium, and tin. This non-toxic alloy is used as a safer substitute for mercury in high-temperature thermometers, as it remains liquid over a broad temperature range. Gallium is also used in a medical context, where certain gallium salts serve as radioactive tracers for imaging procedures.