Electrons fill orbitals in a consistent order, beginning with those:

1. Lower energy states are occupied before higher ones. 2. Electrons first fill the orbitals closest to the nucleus, then continue to fill orbitals of increasing energy further out. 3. The 1s orbital is the closest orbital to the nucleus, and it is always filled first, before any other orbital can be filled. 4. Only after the inner shells are full does a variable number of electrons appear in the outermost shell. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_