Aufbau principle - Wikipedia
https://en.m.wikipedia.org/wiki/Aufbau_principle
The Aufbau principle , also called the Aufbau rule, states that in the ground state of an atom or ion, electrons first fill subshells of the lowest available energy, then fill subshells of higher energy. For example, the 1s subshell is filled before the 2s subshell is occupied. In this way, the electrons of an atom or ion form the most stable electron configuration possible. An example is the configuration 1s 2s 2p 3s 3p for the phosphorusatom, meaning that the 1s sub…
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