As a Metal
In many ways, bismuth behaves like a metal. It is an element that conducts heat and electricity well. This conductivity is a property shared by metals. Bismuth also has a metallic luster, meaning it has a shiny metallic appearance similar to other metals like steel or silver.
Like other metals, bismuth forms alloys readily with other metals. For example, it can combine with tin to form the low-melting bismuth-tin alloy used in some solders and fuses. Its chemical reactivity is also metal-like. Bismuth tarnishes on exposure to air and burns in air to form the yellow oxide bismuth trioxide. These are traits it shares with metallic elements.
So in terms of its conductive, lustrous, alloying, and reactive tendencies, bismuth is decidedly metal-like. This is why it is often classified as a post-transition metal.
As a Crystal
However, bismuth also displays some properties that are quite distinct from metallic behavior. Most notably, in its solid form bismuth does not form a simple close-packed metallic crystal structure like most metals.
Instead, solid bismuth forms crystals with a rhombohedral lattice structure. When bismuth crystallizes, its atoms align in an open framework geometric pattern, leaving gaps between the atoms. This gives bismuth a lower density compared to other metals and causes its crystals to form with stepping pyramidal shapes.
In addition, bismuth has a very low melting point for a metal – only 271°C, whereas most metals melt at over 1000°C. And notably, bismuth expands as it solidifies, the opposite of what metal alloys usually do. These crystalline properties stand in stark contrast to metallic behavior.
Best Described as Metalloid
So bismuth exhibits mixed properties. It is neither a true metal nor a distinct crystal form of an element. This is why bismuth is often classified as a metalloid or semimetal.
Metalloids like bismuth have structures and properties that are intermediate between metals and nonmetals. Bismuth is metallic in some ways but forms distinct crystals more characteristic of a nonmetal.
So, bismuth is best described as a metalloid element, sharing some qualities of metals and some qualities more reminiscent of crystals. Its metallic and crystalline nature make bismuth an element with unique and useful properties. Bismuth's unusual semi-metallic behavior is what makes it well-suited for applications ranging from medicinal compounds to ball bearings, cosmetics, alloys, and more.
