Question

The geometrical form consisting only of a regular array of points in space is called: (a) Unit cell (b) lattice (c) Crystal (d) amorphous solids

Short answer

The correct answer is (b) lattice.

Step-by-step solution

Understanding Definitions

The first step to solve this problem is to know the definition of all the terms given in the options. (a) A unit cell is a smallest repeating unit in a crystal, which reflects the inherent symmetry of the crystal. (b) A lattice is an abstract construct used to explain the symmetrical structure in crystalline solids, it's an array of points showing the arrangement of particles in a crystal. (c) Crystal refers to a solid material whose atoms are arranged in a highly ordered, repeating pattern extending in all three spatial dimensions. (d) Amorphous solids are materials in which there is no long-range order of the positions of the atoms.

Choosing the Correct Term

Having understood the definitions of all the terms given in the options, we can see that the option that best fits the definition given in the problem, i.e., the geometrical form consisting only of a regular array of points in space is a lattice.

Key concepts

Unit Cell

The concept of a 'unit cell' is foundational for understanding the structural organization within crystalline solids. Imagine a unit cell as the basic building block of a crystal; it's the smallest portion of the crystal lattice that can be repeated in three-dimensional space to create the entire structure of the crystal.

Like a 3D pattern that tiles perfectly without gaps or overlaps, the unit cell contains the geometric arrangement of atoms, ions, or molecules that represent the established order within the solid. Multiple unit cells stack like bricks in a wall to construct the extensive, repeating pattern that we recognize as a crystal.

Crystalline Solids

Crystalline solids, such as table salt or diamonds, are known for their orderly and repetitive arrangement of components. What defines these materials is their long-range ordered structure, where the positions of atoms, ions, or molecules abide by a specific and regular pattern.

This regularity extends throughout the entire piece of the solid, and it's this pattern that gives crystalline solids their characteristic clear edges, flat surfaces, and unique angles. The precise nature of their atomic arrangement also results in distinct physical properties such as cleavage planes, consistent melting points, and anisotropy, meaning their properties can vary when measured along different directions.

Amorphous Solids

In contrast to the orderly world of crystalline solids, amorphous solids present us with a much more disordered structure. The term 'amorphous' itself suggests a lack of form; these solids do not possess a long-range, regular pattern that extends through the material.

Common examples include glass, plastics, and gels. Their atoms or molecules are arranged pretty much at random or have some degree of short-range order. This randomness leads to amorphous solids having quite different properties from crystals. They don't have clear melting points; instead, they soften over a range of temperatures. Amorphous solids can also be tougher and less brittle, as their disordered arrangement can better absorb impacts without breaking.

Spatial Arrangement of Atoms

The spatial arrangement of atoms is a broad concept that encompasses how atoms are positioned relative to each other in both crystalline and amorphous solids. In crystalline structures, atoms are laid out in a highly ordered, three-dimensional pattern, repeated over and over. This orderly structure is governed by the types of chemical bonds, such as ionic or covalent, and the principles of symmetry.

In amorphous structures, however, the spatial arrangement is much less organized. While there may be some local ordering, any consistent pattern dissipates over longer distances. This difference in the spatial arrangement directly influences the properties of materials, affecting their optical, mechanical, electrical, and thermal characteristics.