Spin Quantum Number

What is Spin Quantum Number?

The fourth quantum number which is introduced to describe the orientation of the electron spin (rotation) in space, is called the spin quantum number. It can be clockwise or anticlockwise. 

It is represented by ‘s’ or ‘ms’.

Table of Contents

• Key Points on Spin Quantum Number
• The Spin of an Electron
• Example :
• Determination of Magnetic Nature
• Related Videos
• Frequently Asked Questions – FAQs

Key points on Spin Quantum Number

• Quantum numbers give complete information about the electron in atom I, e., energy, position, size, shape and orientation of that orbital and the direction of spin. The direction of spin is described by the spin quantum number.
• The electron in an atom not only moves around the nucleus, but also spins about its own axis. This number gives information about the direction of spinning of the electron present in any orbital.
• The spin angular momentum is an intrinsic property, like rest mass and charge.
• The magnitude spin quantum number of an electron cannot be changed.
• The spin may lie in the 2s+1=2 orientation.
• Each type of subatomic particle has fixed spin quantum numbers like 0,1/2, 1, 3/2, …  etc.
• The spin value of an electron, proton, or neutron is 1/2. 
• The particles having half-integral value (1/2, 3/2 …) of spin are called fermions. 
• The particles having an integral value (0,1,2..) of spin are called bosons.

The spin of an Electron

An electron spin (s = ½) can take only two orientations with respect to a specified axis.

• An ‘α’ electron (spin-up) is an electron with ms = +1/2    (it is denoted as ↑)
• A ‘β’ electron (spin-down) is an electron with ms = −1/2   (it is denoted as ↓)

Example :

1. ₈O¹⁶, (Number of electrons is 8)

Electronic configuration of oxygen atom is 1s² 2s² 2p⁴ 

Determination of Magnetic Nature

• This quantum number helps to explain the magnetic properties of the substances.
• A spinning electron behaves like a micromagnet with a definite magnetic moment. If an orbital contains two electrons, then their magnetic moment opposes and cancels each other.
• If the orbitals are fully filled, the net magnetic moment is zero and the substance behaves as diamagnetic (i.e. repelled by the external magnetic field).
• The orbitals are half-filled, the substance has a net magnetic moment and is paramagnetic (i.e. attracted by the external magnetic field) 

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