( ( A ( single ( chlorine atom ) ) ) ( ( ( removes ( ( about 100,000 ) ( ozone molecules ) ) ) ( before ( it ( ( undergoes ( chemical reaction ) ) ( with ( another substance ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (JJ single) (NN chlorine) (NN atom)) (VP (VBZ removes) (NP (QP (RB about) (CD 100,000)) (NN ozone) (NNS molecules)) (SBAR (IN before) (S (NP (PRP it)) (VP (VBZ undergoes) (NP (NN chemical) (NN reaction)) (PP (IN with) (NP (DT another) (NN substance))))))) (. .)))
A single chlorine atom removes about 100,000 ozone molecules before it undergoes chemical reaction with another substance.
(ROOT (S (ADVP (RB Replace)) (NP (NP (DT another)) (PP (IN in) (NP (DT a) (NN molecule)))) (VP (VBZ happens) (PP (TO to) (NP (NNS atoms))) (PP (IN during) (NP (DT a) (NN substitution) (NN reaction)))) (. .)))
Replace another in a molecule happens to atoms during a substitution reaction.