( ( ( The electrons ) ( in ( ( the ( outermost ( energy level ) ) ) ( of ( an atom ) ) ) ) ) ( ( are ( referred ( to ( ( as valence ) electrons ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NNS electrons)) (PP (IN in) (NP (NP (DT the) (JJ outermost) (NN energy) (NN level)) (PP (IN of) (NP (DT an) (NN atom)))))) (VP (VBP are) (VP (VBN referred) (PP (TO to) (NP (ADJP (RB as) (JJ valence)) (NNS electrons))))) (. .)))
The electrons in the outermost energy level of an atom are referred to as 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)))) (. .)))