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Al E.
Dec 21, 2017

It depends on how detailed you'd like me to get.

In short: due to quantum physics.

Each orbital is a cloud of space representing the probabilistic location of two electrons. One of these electrons generally spins "up" and the other generally spins "down".

In the presence of a magnetic field, these up- and down-spin electrons exhibit characteristic energy.

Principles of General Chemistry; Averill, et al.

Other subatomic particles have these spins, as well. When chemists draw orbital shorthand, they generally fill in the #uparrow# in every orbital before the #downarrow#. I remember my professor saying this is convention; but there may be a more complicated, interesting reason which I don't know (Hund's rule may imply this?).

Chemistry Libretexts

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