Aufbau principle

And this is why the periodic table is shaped the way it is, with different blocks and regions reflecting these orbital structures.

Electrons fill these orbitals following what's called the Aufbau principle, which says they fill in order of increasing energy, like water filling containers from the lowest point upward.

Each orbital can hold a maximum of two electrons, and these two electrons must have opposite spins, a property that's somewhat analogous to rotation, though not exactly, since electrons are quantum objects and don't actually rotate.

There's also something called Hund's Rule, which says that when you have multiple orbitals at the same energy level, electrons will fill them one at a time with parallel spins before they start pairing up.