Most are aware that electrons are negatively charged particles that surround the nucleus of atoms and whose behaviour governs chemical interactions. However, it is less commonly known that electrons come in two distinct kinds: spin-up and spin-down. And the tendency for pairing between up and down spin electrons, forming "dance partners" with one another, is one of the most important behaviours affecting the electron clouds that control the chemistry of nature. Under pressures like those deep inside the Earth, the orbits in which the electrons move are squeezed, the "dance floor" changes. Electron pairs are sometimes forced to change their dance pattern and the way that they partner with one another, giving rise to what is termed an "electron spin-pairing crossover" ("spin crossover" is often used as a shorthand expression).
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