In atomic physics, the spin quantum number is a quantum number (designated ) which describes the intrinsic angular momentum (or spin angular momentum, or simply spin) of an electron or other particle. The phrase was originally used to describe the fourth of a set of quantum numbers (the principal quantum number , the azimuthal quantum number , the magnetic quantum number , and the spin quantum number ), which completely describe the quantum state of an electron in an atom. The name comes from a physical spinning of the electron about an axis, as proposed by Uhlenbeck and Goudsmit. The value of is the component of spin angular momentum parallel to a given direction (the –axis), which can be either +1/2 or –1/2 (in units of the reduced Planck constant). However this simplistic picture was quickly realized to be physically impossible because it would require the electrons to rotate faster than the speed of light. It was therefore replaced by a more abstract quantum-mechanical description. This description technically involves two spin quantum numbers and , where is related to the magnitude of the electron spin. However is always +1/2 for an electron, so it is not necessary to include its value in the set of quantum numbers describing the state of each electron in an atom. At an elementary level, is described as the spin quantum number, and is not mentioned since its value 1/2 is a fixed property of the electron. At a more advanced level where quantum mechanical operators are introduced, is referred to as the spin quantum number, and is described as the spin magnetic quantum number or as the z-component of spin . Key points Quantum numbers give complete information about the electron in an atom, i.e., energy, position, size, shape and orientation of that orbital and the direction of spin. The direction of spin is described by spin quantum number. The electron in an atom not only moves around the nucleus, but also spins about its own axis. This number gives the 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 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