- What is the difference between a crystalline solid and an amorphous solid?
Crystalline solids have a well-defined and repeating pattern of atoms or molecules arranged in a regular and orderly manner in three-dimensional space. They have a well-defined lattice structure, and their atoms or molecules are held together by strong intermolecular forces. Crystalline solids have a characteristic melting point and exhibit sharp diffraction patterns in X-ray or electron diffraction experiments.
On the other hand, amorphous solids lack a well-defined lattice structure and have a disordered arrangement of atoms or molecules. They do not have a sharp melting point and typically soften gradually over a range of temperatures. Amorphous solids exhibit broad diffraction patterns in X-ray or electron diffraction experiments.
Examples of crystalline solids include diamond, salt, and quartz, while examples of amorphous solids include glass, rubber, and some plastics.
- Define the term 'unit cell'.
A unit cell is the smallest repeating unit of a crystal lattice. It is a three-dimensional structure that contains one or more atoms or molecules and has the same symmetry as the overall crystal lattice. By repeating the unit cell in three dimensions, we can construct the entire crystal lattice. The properties of the crystal lattice, such as its mechanical, electrical, and thermal properties, are determined by the arrangement of the atoms or molecules in the unit cell.
- What is the difference between a primitive cell and a unit cell?
A primitive cell is the smallest repeating unit of a crystal lattice that contains only one lattice point. It is a subset of a unit cell and is characterized by having the minimum possible number of atoms or molecules that can be used to construct the crystal lattice. A primitive cell can have a simple or complex shape, depending on the symmetry of the crystal lattice.
A unit cell, on the other hand, is the smallest repeating unit of a crystal lattice that contains all the information about the symmetry and arrangement of the atoms or molecules in the crystal lattice. A unit cell can contain one or more lattice points and can be divided into smaller primitive cells.
- What is the coordination number of atoms in a body-centered cubic (BCC) crystal structure?
In a body-centered cubic (BCC) crystal structure, each atom is surrounded by eight neighboring atoms, which are located at the corners of a cube, as well as one atom located at the center of the cube. The coordination number of atoms in a BCC crystal structure is therefore 8 + 1 = 9.
- Define the term 'packing efficiency'.
The packing efficiency is a measure of how efficiently atoms or molecules are packed together in a crystal lattice. It is defined as the ratio of the volume occupied by the atoms or molecules in the unit cell to the total volume of the unit cell. The packing efficiency can range from 0 to 1, with higher values indicating more efficient packing.
For example, in a face-centered cubic (FCC) crystal lattice, the packing efficiency is 0.74, which means that 74% of the volume of the unit cell is occupied by the atoms. In a body-centered cubic (BCC) crystal lattice, the packing efficiency is 0.68, which means that 68% of the volume of the unit cell is occupied by the atoms. The highest possible packing efficiency is 1, which is achieved in a close-packed structure such as the hexagonal close-packed (HCP) structure.