Aufbau principle - Wikipedia
https://en.wikipedia.org/wiki/Aufbau_principle
WEBThe Aufbau principle (/ ˈ aʊ f b aʊ /, from the German Aufbauprinzip, which means "building-up 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 ...
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