( Atoms ( ( are ( ( most stable ) ( when ( they ( have ( ( eight electrons ) ( in ( their ( outer ( electron ( energy level ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Atoms)) (VP (VBP are) (ADJP (ADJP (RBS most) (JJ stable)) (SBAR (WHADVP (WRB when)) (S (NP (PRP they)) (VP (VBP have) (NP (NP (CD eight) (NNS electrons)) (PP (IN in) (NP (PRP$ their) (JJ outer) (NN electron) (NN energy) (NN level))))))))) (. .)))
Atoms are most stable when they have eight electrons in their outer electron energy level.
(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)))) (. .)))