Wikipedia
In quantum mechanics, a triplet is a quantum state of a system with a spin of 1, such that there are three allowed values of the spin component: −1, 0 and +1.
Spin, in the context of quantum mechanics, is not a mechanical rotation but a more abstract concept that characterizes a particle's intrinsic angular momentum. It is particularly important for systems at atomic length scales, such as individual atoms, protons, or electrons.
Almost all molecules encountered in daily life exist in a singlet state, but molecular oxygen is an exception. At room temperature, O exists in a triplet state, which can only undergo a chemical reaction by making the forbidden transition into a singlet state. This makes it kinetically nonreactive despite being thermodynamically a strong oxidant. Photochemical or thermal activation can bring it into the singlet state, which makes it kinetically as well as thermodynamically a strong oxidant.
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