( ( ( Energy levels ) ( of ( electrons ( in ( an atom ) ) ) ) ) ( ( are quantized ) . ) )
(ROOT (S (NP (NP (NNP Energy) (NNS levels)) (PP (IN of) (NP (NP (NNS electrons)) (PP (IN in) (NP (DT an) (NN atom)))))) (VP (VBP are) (VP (VBN quantized))) (. .)))
Energy levels of electrons in an atom are quantized.
(ROOT (S (NP (NP (NNS Electrons)) (PP (IN in) (NP (NP (DT the) (JJ outer) (NN energy) (NN level)) (PP (IN of) (NP (DT an) (NN atom)))))) (VP (VBP are) (VP (VBN called) (NP (JJ valence) (NNS electrons)))) (. .)))