Question

The most malleable metals (Cu, Ag, Au) have close-packing of the type : (a) Hexagonal close-packing (b) Cubic close-packing (c) Body-centred cubic packing (d) Malleability is not related to type of packing

Short answer

The most malleable metals (Cu, Ag, Au) have close-packing of the type (b) Cubic close-packing.

Step-by-step solution

Understanding Metal Packing and Malleability

The malleability of a metal is related to its ability to deform under compressive stress. Metals that can readily deform without breaking are considered malleable. Close packing in metals allows layers of atoms to slide over each other easily which contributes to malleability.

Identifying the Closest Packed Structures

There are two primary types of closest packed structures: hexagonal close-packing (hcp) and cubic close-packing (ccp). In both of these structures, each atom is surrounded by 12 other atoms, achieving the maximum packing efficiency.

Determining the Packing Type for Cu, Ag, Au

Copper (Cu), Silver (Ag), and Gold (Au) are known for their exceptional malleability and they adopt the cubic close-packing structure also referred to as face-centered cubic (fcc) structure. This structure allows layers of atoms to slide over each other with relative ease.

Key concepts

Hexagonal Close-Packing

Hexagonal close-packing (hcp) is one of the ways in which atoms can be arranged in a solid material. Atoms in hcp structures are packed together as closely as possible, with each atom surrounded by 12 others, just like the arrangement of spheres in a tight pyramid-like structure.

The hcp structure is characterized by its layers: each layer of atoms is positioned directly above and below the gaps in the layers sandwiching it. This layered arrangement forms a repeating ABAB pattern. Despite being one of the densest packings possible, hcp metals are generally less malleable than their cubic close-packed counterparts because the sliding planes are less symmetrical and fewer in number, limiting the ways in which layers can slide over each other under stress.

Cubic Close-Packing

Cubic close-packing (ccp), also known as face-centered cubic (fcc) packing, involves a different arrangement where each atom is also surrounded by 12 others, but in a way that results in a repeating ABCABC pattern. This structure is found in highly malleable metals such as Copper (Cu), Silver (Ag), and Gold (Au), as mentioned in the textbook exercise.

The fcc structure is significant for malleability because it has multiple slip planes, or layers that can move relative to each other, which allows these materials to be deformed under compressive forces without fracturing. This characteristic explains why fcc metals can be hammered into thin sheets, a property highly valued in applications that require shaping metals through mechanical processing.

Compressive Stress in Metals

Compressive stress in metals refers to the force applied to a metal that attempts to squeeze or compress the material. Metals respond to compressive stress by deforming, and their ability to do so without cracking or shattering is linked to their atomic arrangement.

When stress is applied, the planes of atoms in a metal can slide past one another, a process known as plastic deformation. Metals with high malleability, such as those with cubic close-packing, are better suited to withstand these stresses, which aligns with their use in industries that require metal bending, pressing, or hammering. Understanding the resistance to compressive stress is essential for materials engineering, as it helps predict a metal's behavior under load and informs decisions on metal selection for various applications.

Malleable Metals

Malleable metals are those that can be hammered, pressed, or rolled into thin sheets without breaking. Their malleability is crucial for crafting items such as wires, sheets, and various metal components. The textbook exercise highlights Copper, Silver, and Gold, all known for this property, and all exhibiting cubic close-packing.

The underlying atomic structure of malleable metals is a key factor in their ability to deform. The ccp structure has the highest degree of symmetry and the greatest number of slip planes, therefore facilitating the ease with which these metals can absorb and redistribute compressive stress. As a result, craftsmen and manufacturers can form malleable metals into desired shapes with relative ease, making them invaluable in the production of electronics, jewelry, and industrial parts.