This property of atomic electrons was discovered through experimentation. The close observation of spectral lines revealed that each line was actually a pair of very closely-spaced lines, and what is referred to as fine structure was hypothesized to result from each electron “spinning” on an axis as if a planet. Electrons with different “spins” would give off slightly different frequencies of light when excited. The name “spin” was assigned to this quantum number.
Spin quantum numbers are symbolized as ms in atomic physics and sz in nuclear physics. For each orbital in each subshell in each shell, there may be two electrons, one with a spin of + ½ and the other with a spin of – ½.
The value ms = + ½ refers to the spin-up case, sometimes designated with an up arrow (↑) or simply a plus sign (+). In similar way, ms = – ½ is the spin-down case (↓) or (-).
Related: Magnetic Quantum Number
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