Unit Cell

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What is a Unit Cell?

A unit cell is said to be the smallest repeating unit of the crystal lattice in the living organism, which is also said to be the building block of the crystal. That’s how life started to exist on earth. Atoms are arranged in a three-dimensional symmetry within a crystal structure. There are three types of unit cells present in nature, primitive cubic, body-centered cubic, and face-centered cubic. 


Types of Unit Cell

As we have already mentioned earlier, there are three types of unit cells, and we will discuss them in brief in this section.


Primitive Cubic Unit Cell 

If you look at the primitive cubic unit cell, you will find out that only at the corners, you are going to find the atoms. Every single atom which is present in the unit cell is shared with the other adjacent cell. Meaning each unit cell shares its atoms with 8 other primitive cubic unit cells. Thus, a particular unit cell contains only ⅛ of an atom. On the other hand, 4 unit cells are present in the same layer, and there are 2 layers. The upper and the lower layer makes a particular unit cell to be 1/8th of the atom that is present inside it. 

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The total number of atoms present in the single primitive cubic unit cell can be counted by taking eight atoms present in the corner. Thus, ⅛ X 8 = 1 atom.


Body-centered Cubic Unit Cell 

The BCC is almost the same as a simple cubic unit cell, meaning it has eight atoms present in each corner of the cube and one atom in the center of the cube. BCC has an opened structure. The atom which is present at the center solely belongs to the unit cell in which it is present. 

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To find the number of atoms present in BCC, you need to consider two things.

First, 8 corners will make ⅛ per corner atom = 8 x ⅛ = 1 atom

One atom present at the center of the body = 1 atom

Thus, the body-centered cubic unit cell has two atoms. 


Face-centred Cubic Unit Cell (FCC)

A face-centered cubic unit cell is the most densely populated unit cell. In addition to this, at the center of the cube's faces, you will find out one atom. These face-centered atoms are shared between 2 adjacent unit cells, and only ½ of each atom is a part of the individual cell. 

To find out the number of atoms present in the FCC, you need to know this in the first place.

There are 8 corners x 18 per corner atom = 8 x 18 = 1 atom 

6 face-centered atoms x 12 atoms per unit cell = 3 atoms 

Thus, we have 4 atoms in a Face centered cubic unit cell.


What is the Basic Unit of Life?

What distinguishes a living organism from a non-living object? Well, the answer is a living being will have self-sustaining biological processes. A cell is the smallest and the most basic form in which life exists on earth. The new cells made in the living organism came into existence from the division of the preexisting cells into two.  

FAQ (Frequently Asked Questions)

Q1. What is a Primitive Unit Cell?

Ans: A primitive cell is said to be the cell unit, which contains exactly one lattice point. In terms of size, it is the smallest possible cell. On the other hand, if there is a lattice point present at the cell's very edge, and it is shared with another cell, it will only count on behalf. Moreover, if a point is located on the corner of the cube, it is shared by 8 cubes, and the count would be ⅛.


A cell will always try to fill the lattice space without leaving any space with crystal translation operations. 

Q2. What is the Difference Between a Space Lattice and a Unit Cell?

Ans: A space lattice is a map of an array of points drawn to show how particles such as ions, atoms, and molecules arrange themselves at different sites when presented in three-dimensional spaces. 


When it comes to defining unit cells, you can say it is the smallest repeating unit in space lattice, which will give you a crystal of the given substance if you repeat again and again over time. Every single unit cell needs its vectors a and b for its description. A three-dimensional space lattice also is divided into smaller unit cells, which can be described by three vectors.