( ( All ( chemical reactions ) ) ( ( occur ( between ( electrons ( in ( ( the ( outer ( energy level ) ) ) ( of atoms ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT All) (NN chemical) (NNS reactions)) (VP (VBP occur) (PP (IN between) (NP (NP (NNS electrons)) (PP (IN in) (NP (NP (DT the) (JJ outer) (NN energy) (NN level)) (PP (IN of) (NP (NNS atoms)))))))) (. .)))
All chemical reactions occur between electrons in the outer energy level of atoms.
(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)))) (. .)))