Question

Question #0ba2f

Answer 1

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.

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?).