The *Elementary Particles *, *Composite Particles & Atomic Nuclei* have a property called “*Spin”*. Spin can be defined as ,

The Angular Momentum Which Is Possessed By Any Of The Sub-Atomic Particles Or Elementary Particles Is Called The “Spin” Of That Particle.

The Angular Momentum of a particle are of Two types :

*Spin Angular Momentum Or Spin.**Orbital Angular Momentum.*

In this section of my post, I will be discussing only on the *Spin Angular Momentum* Or *Spin* of a particle.

One way of thinking of Spin of particles, Is to imagine the particles as little *“Tops”*, Spinning about an axis. But this could be mis-leading because *Quantum Mechanics* tells us that the particle do not have any well defined axis.

All the Elementary Particles have some sort of *Angular Momentum*. That Angular Momentum will have some sort of *Magnitude*. Same kind of Elementary Particle will have the same Magnitude of *Spin Angular Momentum*. Thus, As per the Magnitude of Angular Momentum, Elementary Particles are assigned by the *Spin Quantum Number*, Which indicate the Magnitude of Angular Momentum of that particular Elementary Particle.

We can calculate the Spin Quantum Number with the formula :

*Where,*

**s** is **Spin Quantum Numbe**r

* n is any Non - Negative Integer*

*We can calculate the Spin Quantum Number of a particle by putting the values in the above equation, For example*

*Let, n = 0*

*s = n / 2*

*s = 0 / 2*

**s = 0**

*Let, n = 1*

**s = 1/2**

*Let, n = 2*

**s = 1**

*Let, n = 3*

**s = 3/2**

*Let, n = 4*

**s = 2**

*And So On.*

Thus, We get the values of s as, 0, 1/2, 1, 3/2, 2 & so on. We cannot make an Elementary Particle to Spin *Faster* or* Slower*. But the *Direction* of Spin of an Elementary Particle can be changed. The value of s depends only on the type of the Elementary Particle & the value of s cannot be changed.

The *Spin of a particle* tells us that what a particle looks like from different directions. Let us study some of the examples of how a particle will look from different directions.

Let us consider a particle of *Spin 0 or s = 0*, So the particle of Spin 0 will look like a *“Dot”*. As the Dot looks same from every where, The particle of Spin 0 will also look same from every direction.

Now the particle of *Spin 1 or s = 1* will look like an *Arrow*, As the Arrow look different from every direction, Similarly the particle of Spin 1 will also look different from different direction. Only if one turns round a complete revolution of *360 Degrees*, Then the particle looks the same.

Let us consider a particle of *Spin 2 or s = 2*, The particle of Spin 2 will look like a *Double - Headed Arrow*. It also looks different from different directions. But it looks same if one turns it round half a revolution of *180 degrees *.

Similarly, *Higher Spin particles* looks the same if one turns them through *Small Fractions* of one complete revolution. But the extraordinary fact is that, *“The particles do not looks the same if one turns them through just One Revolution. Hence, We have to turns them through Two Complete Revolutions”*. Such Particles are said to have *Spin 1/2 or s = 1/2 *.

The particles which are known in the Universe can be divided into two groups:

*The particles which make up the Matter in the Universe are particles of Spin 1/2 or s = 1/2. These particles are also known as “Fermions".**The particles which give rise to forces between Matter particles are of Spin 0, 1 & 2 or s= 0, 1 & 2. These particles are also known as “Bosons” .*

The particles of *Spin 1/2* or *“**Fermions”* follows sir *“Pauli's Exclusion Principle”*.

Pauli’s Exclusion Principle Says That, Two Similar Particles Or“Fermions” Cannot Exist In Same State, i.e. They Cannot Have Both, The Same“ Position”And Same“Velocity” At Same “Time”, Within The Limits Given By Uncertainty Principle.

The *Pauli's Exclusion Principle* is very important because it explains why *Matter Particles* do no collapse to form a state of very high density under the influence of the forces produced by the particles of *Spin 0, 1, 2 or Bosons*. If the particles of Spin 0, 1, 2 or Bosons have same *Position* as well as same *Velocity*, Then the particles would all collapse to form a *Uniform Dense Matter*.

According to the above statement, The particles will nearly have same Positions but they must have different Velocities. They will not stay in same position for long time. Thus, Without *Pauli's Exclusion Principle*, *Quarks* would not form separate, Well-Defined *Protons & Neutrons *.

The Pauli's Exclusion Principle was first discovered in the year of *1925* by Sir *“Wolfgang Pauli”*. He was honoured with the Nobel Prize for the discovery of the *Exclusion Principle*, Also known as “*Pauli's Exclusion Principle”*.

Thus, *Spin* of an Elementary Particle is a very important factor, Which also determine the *Shape* & the *Size* of *Elementary* or *Matter Particles*. Each and every objects around us which we can observe or feel or those objects which are not observable with our naked eyes are composed of Elementary Particles & these Elementary Particles have some sort of *Spin*. Thus, We can say that the Spin defines our *" Building Blocks Of Nature "*.