When several orbitals share the same energy, electrons occupy them:

1. Electrons repel one another, so they spread out where they can. 2. If there are multiple orbitals of equal energy, they will be filled with one electron in each energy level before a second electron is added. 3. When filling the p orbitals, each takes a single electron; once each p orbital has an electron, a second may be added. 4. Pairing from the start would put two mutually repelling electrons in one small region unnecessarily. 5. This is why half-filled subshells appear part way through a period. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_