11.17
Metallic Solids
Metallic solids such as crystals of copper, aluminum, and iron are formed by metal atoms. The structure of metallic crystals is often described as a u…
Metallic solids, such as copper, aluminum, and gold, are solids that have metal atoms as their constituent particles, which typically are held together by strong metallic bonds.
In a metallic bond, the electron clouds of neighboring metal atoms overlap, such that these valence electrons are delocalized throughout the solid. Metallic solids are sometimes described as a lattice of stationary metal cations in a sea of delocalized electrons.
This results in a more flexible internal structure than that of molecular or ionic solids. Metallic solids generally can bend and deform without breaking to a greater extent than other solids.
As exhibited by gold and mercury, their melting points vary widely depending on the strength of the metallic bonds. The mobile electrons also make these solids excellent conductors of heat and electricity.
The atoms of pure metallic solids are typically arranged, or ‘packed’, in one of the two most efficient possible configurations for spheres of a uniform size: hexagonal close-packing or cubic close-packing.
In hexagonal close-packing, the metal atoms are arranged hexagonally in two repeating layers, where the atoms in the second layer occupy the depressions of the first layer. The third layer is a repetition of the first layer, resulting in a characteristic ABAB arrangement.
Zinc metal exhibits a hexagonal close-packed, or hcp, lattice. Its unit cell contains two atoms that are not on the lattice points.
In cubic close-packing, the atoms are arranged hexagonally in three repeating layers. The atoms in every fourth layer align with the atoms in the first layer, making a characteristic ABCABC arrangement.
Copper metal has a cubic close-packed, or ccp, lattice. This lattice is also called face-centered cubic, or fcc, owing to the arrangement being that of a face-centered cubic lattice with an identical atom on every lattice point.
In both types of packing, each atom has a coordination number of twelve. In hcp stacking, each atom is in direct contact with six atoms from the same layer, three from the layer above, and three from the layer below.
View the full transcript and gain access to JoVE Core videos
Q1: What is a metallic bond and how does it differ from other types of chemical bonds?
A metallic bond forms when electron clouds of neighboring metal atoms overlap, causing valence electrons to become delocalized throughout the solid. Unlike ionic or covalent bonds, metallic bonding creates a flexible lattice of stationary metal cations surrounded by a sea of mobile electrons. This unique arrangement allows metals to conduct electricity and heat exceptionally well while remaining malleable and ductile.
Q2: Why do metallic solids conduct electricity and heat so effectively?
Metallic solids conduct electricity and heat due to their delocalized electrons, which move freely throughout the structure. These mobile electrons can transfer energy and charge rapidly across the material without being bound to specific atoms. This electron mobility is a direct consequence of metallic bonding and explains why metals like copper and aluminum are preferred for electrical wiring and heat transfer applications.
Q3: What is the difference between hexagonal close-packing and cubic close-packing in metals?
Hexagonal close-packing (hcp) arranges metal atoms in alternating ABAB layers, where the third layer repeats the first layer's position. Cubic close-packing (ccp) uses three repeating ABCABC layers, with the fourth layer aligning with the first. Both achieve 74% volume efficiency and give each atom a coordination number of twelve, but they differ in layer stacking patterns. Zinc exhibits hcp structure, while copper displays ccp structure.
Q4: Why can metallic solids bend and deform without breaking?
Metallic solids can bend and deform because their flexible internal structure allows layers of atoms to slide past one another while maintaining metallic bonding. The delocalized electron sea accommodates atomic displacement without breaking bonds, unlike rigid ionic or molecular solids. This malleability makes metals valuable construction materials, as they absorb stress through deformation rather than fracturing.
Q5: How do metallic bonding strengths explain the wide variation in melting points among metals?
Metallic bonding strength varies significantly among metals, directly affecting their melting points. Mercury is liquid at room temperature due to weak metallic bonds, while transition metals like tungsten melt above 1000°C because of strong metallic interactions. Alkali metals melt below 200°C, reflecting intermediate bond strengths. These differences in bonding strength determine how much thermal energy is required to disrupt the metallic lattice.
Q6: What is the coordination number in metallic solids and why is it significant?
The coordination number in metallic solids is twelve, meaning each atom contacts six atoms in its own layer, three in the layer above, and three in the layer below. This high coordination number is significant because it maximizes attractive forces between atoms and minimizes total intermolecular energy, making close-packing arrangements the most stable and common configuration in metals.
Q7: What properties make metallic solids useful for construction and industrial applications?
Metallic solids exhibit high thermal and electrical conductivity, metallic luster, malleability, and variable hardness. Their malleability allows them to deform under pressure without shattering, making them ideal for construction. Many metals are also very strong, and their ability to conduct heat and electricity efficiently makes them suitable for diverse industrial applications from building frameworks to electrical systems.