( ( ( ( ( ( The ( outer shell ) ) ( of ( an atom ) ) ) ( is ( called ( the ( valence shell ) ) ) ) ) and ) ( ( its electrons ) ( are ( called ( valence electrons ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (JJ outer) (NN shell)) (PP (IN of) (NP (DT an) (NN atom)))) (VP (VBZ is) (VP (VBN called) (NP (DT the) (JJ valence) (NN shell))))) (CC and) (S (NP (PRP$ its) (NNS electrons)) (VP (VBP are) (VP (VBN called) (NP (JJ valence) (NNS electrons))))) (. .)))
The outer shell of an atom is called the valence shell and its electrons are called valence electrons.
(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)))) (. .)))