Which unit cell is represented when constituent particles are present only on the corner positions of a unit cell?

Primitive Unit Cell

The correct answer is option 4. Primitive Unit Cell.

In crystallography, a unit cell is the smallest repeating unit of a crystal lattice, and it represents the arrangement of atoms, ions, or molecules in the crystal structure. There are several types of unit cells, each with its own unique arrangement of constituent particles.

1. Primitive Unit Cell: In a primitive unit cell, constituent particles (atoms, ions, or molecules) are present only at the corner positions of the unit cell. Each corner of the unit cell contains one constituent particle. The constituent particles are not shared between adjacent unit cells; they belong exclusively to the unit cell in which they are located. The primitive unit cell is the simplest and most basic type of unit cell. This type of unit cell is characteristic of structures with simple cubic (sc), body-centered cubic (bcc), or face-centered cubic (fcc) crystal lattices, where each lattice point represents one constituent particle.

2. Body-Centered Unit Cell: In a body-centered unit cell, in addition to the constituent particles at the corner positions, there is also one constituent particle at the center of the unit cell. The body-centered unit cell is commonly found in body-centered cubic (bcc) crystal lattices.

3. Face-Centered Unit Cell: In a face-centered unit cell, in addition to the constituent particles at the corner positions, there is also one constituent particle at the center of each face of the unit cell. The face-centered unit cell is commonly found in face-centered cubic (fcc) crystal lattices.

4. End-Centered Unit Cell: In an end-centered unit cell, in addition to the constituent particles at the corner positions, there is also one constituent particle at the center of two opposite faces of the unit cell. The end-centered unit cell is less common and is found in some crystal structures, such as monoclinic and orthorhombic lattices.

Therefore, when constituent particles are present only on the corner positions of a unit cell, the type of unit cell represented is the Primitive Unit Cell. It is characterized by its simplicity and the exclusive presence of particles at the corners.