Intermediate Topic 56 of 1102 Atomic Structure & Periodicity
Spin Quantum Number
The number m_s, either +1/2 or -1/2, describing the intrinsic spin of an electron.
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In-depth explanation of Spin Quantum Number
The spin quantum number, m_s, describes the intrinsic angular momentum of an electron, and it can take only the values +1/2 or -1/2, often called spin up and spin down. Electrons behave as if they are spinning, although this is a quantum property with no classical equivalent. Two electrons in the same orbital must have opposite spins, according to the Pauli principle. Unpaired electrons make an atom paramagnetic, while fully paired electrons give diamagnetic behaviour. Spin was demonstrated directly by the Stern-Gerlach experiment in 1922.
Going deeper: Spin Quantum Number
Key ideas: ms is +1/2 or -1/2, representing two spin orientations of an electron (up and down), with spin angular momentum sqrt(3) / 2 x h / (2 pi). It was shown by Stern-Gerlach (1922) using silver atoms. Each orbital holds at most two electrons, which must have opposite ms. Unpaired spins cause paramagnetism.
Common mistakes and connections: Students believe the electron literally spins like a top; spin is an intrinsic quantum property with no classical analogue. Another error is reading arrows as the exact size of the spin. Spin underlies the Pauli principle, Hund's rule, magnetic moment mu = sqrt(n (n + 2)) Bohr magnetons for n unpaired electrons.
Visualization of Spin Quantum Number
In a 1 T field the two electron spin states differ by about 0.116 meV, the splitting used in electron spin resonance. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Helium's two 1s electrons are paired with opposite spins, so helium is diamagnetic.
Frequently asked questions about Spin Quantum Number
What is Spin Quantum Number?
The number m_s, either +1/2 or -1/2, describing the intrinsic spin of an electron.
Is Spin Quantum Number a beginner, intermediate or advanced topic?
Spin Quantum Number is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
What should I learn before Spin Quantum Number?
Study these earlier topics in Atomic Structure & Periodicity first: Principal Quantum Number, Azimuthal Quantum Number, Magnetic Quantum Number.
What comes after Spin Quantum Number?
The next topics in Atomic Structure & Periodicity are Shapes of s and p Orbitals and Shapes of d and f Orbitals.
Can I practise Spin Quantum Number with quizzes?
Yes. The interactive lesson includes a self-check question on Spin Quantum Number, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Where Spin Quantum Number fits in Atomic Structure & Periodicity
Spin Quantum Number is topic 56 of 115 in Atomic Structure & Periodicity.