Arrangement of electrons is done according to the principle proposed by Aufbau. Electrons must be filled
according to the arrow marks as shown in the figure. The Aufbau principle works for nearly every element discovered. There are
few exceptions to this principle, chromium and copper deviate from it.
Aufbau comes from the German word “Aufbauen” which means “to build”. In essence when writing electron configurations we are building up electron orbitals as we proceed from atom to atom. As we write the electron configuration for an atom, we will fill the orbitals in order of increasing atomic number. The Aufbau principle originates from the Pauli's exclusion principle which says that no two electrons in an atom can have the same set of quantum numbers, hence they have to “pile up” or “build up” into higher energy levels. How the electrons build up is a topic of electron configurations.
According to the Aufbau principle, electrons fill orbitals starting at the lowest available (possible) energy levels before filling higher levels (e.g. 1s before 2s).
If we follow the pattern across a period from B (Z=5) to Ne (Z=10) the number of electrons increase and the sub shells are filled. Here we are focusing on the p sub shell in which as we move towards Ne, the p sub shell becomes filled.
- O (Z=8) configuration: 1s22s22p4
- F (Z=9) configuration: 1s22s22p5
- Ne (Z=10) configuration: 1s22s22p6