When we face first time with Quantum Mechanics one of the new concept to understand is how works the

**Angular Momentum and the Spin of a Particle**.

In quantum mechanics and particle physics,

**spin is an intrinsic form of angular momentum carried by elementary particles**, composite particles (hadrons), and atomic nuclei.

Spin is one of two types of

**angular momentum in quantum mechanics**, the other being orbital angular momentum. Orbital angular momentum is the quantum-mechanical counterpart to the classical notion of angular momentum: it arises when a particle executes a rotating or twisting trajectory (such as when an electron orbits a nucleus).

The existence of spin angular momentum is inferred from experiments, such as the

**Stern–Gerlach experiment**, in which particles are observed to possess angular momentum that cannot be accounted for by orbital angular momentum alone. In some ways, spin is like a vector quantity; it has a definite magnitude, and it has a "direction" (but quantization makes this "direction" different from the direction of an ordinary vector).

**All elementary particles of a given kind have the same magnitude of spin angular momentum**, which is indicated by assigning the particle a spin quantum number. The SI unit of spin is the joule-second, just as with classical angular momentum. In practice, however, it is written as a multiple of the reduced Planck constant ħ, usually in natural units, where the ħ is omitted, resulting in a unitless number.

**Spin quantum numbers are unitless numbers by definition**.
When combined with the spin-statistics theorem, the spin of electrons results in the

**Pauli exclusion principle**, which in turn underlies the

**periodic table of chemical elements**.

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Video. Angular Momentum and Spin

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