( ( If ( ( an electron ) ( is ( unpaired then ) ) ) ) ( ( another ( ( atom or ) molecule ) ) ( ( can ( ( easily bond ) ( to ( it ( causing ( a ( chemical reaction ) ) ) ) ) ) ) . ) ) )
(ROOT (S (SBAR (IN If) (S (NP (DT an) (NN electron)) (VP (VBZ is) (VP (VBN unpaired) (ADVP (RB then)))))) (NP (DT another) (NN atom) (CC or) (NN molecule)) (VP (MD can) (VP (VB easily) (NP (NN bond)) (PP (TO to) (S (NP (PRP it)) (VP (VBG causing) (NP (DT a) (JJ chemical) (NN reaction))))))) (. .)))
If an electron is unpaired then another atom or molecule can easily bond to it causing a chemical reaction.
(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.