( ( ( The ( rules af ) ) ( ( placing electrons ) ( within shells ) ) ) ( ( is ( known ( as ( the ( Aufbau principle ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NNS rules) (NN af)) (VP (VBG placing) (NP (NNS electrons)) (PP (IN within) (NP (NNS shells))))) (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (DT the) (NNP Aufbau) (NN principle))))) (. .)))
The rules af placing electrons within shells is known as the Aufbau principle.
(ROOT (S (NP (NNP Aufbau) (NN principle)) (VP (VBZ gives) (NP (NP (DT the) (NN order)) (PP (IN of) (NP (NP (NN electron)) (VP (VBG filling) (PP (IN in) (NP (DT an) (NN atom)))))))) (. .)))
Aufbau principle gives the order of electron filling in an atom.