( ( At times ) ( it ( ( is ( ( ( the ( unshared pairs ) ) ( of ( electrons ( from ( a ( chlorine atom ) ) ) ) ) ) ( which ( will ( ( ( react , ) ( not ( ( ( ( with ( one ( of ( the ( hydrogen atoms ) ) ) ) ) , ) but ) ( with ( the ( cationic ( carbon atom ) ) ) ) ) ) ) itself ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN At) (NP (NNS times))) (NP (PRP it)) (VP (VBZ is) (NP (NP (DT the) (JJ unshared) (NNS pairs)) (PP (IN of) (NP (NP (NNS electrons)) (PP (IN from) (NP (DT a) (NN chlorine) (NN atom))))) (SBAR (WHNP (WDT which)) (S (VP (MD will) (VP (VB react) (, ,) (PP (RB not) (PP (IN with) (NP (NP (CD one)) (PP (IN of) (NP (DT the) (NN hydrogen) (NNS atoms))))) (, ,) (CC but) (PP (IN with) (NP (DT the) (JJ cationic) (NN carbon) (NN atom)))) (NP (PRP itself)))))))) (. .)))
At times it is the unshared pairs of electrons from a chlorine atom which will react, not with one of the hydrogen atoms, but with the cationic carbon atom itself.
(ROOT (S (NP (EX There)) (VP (VBP are) (NP (NP (CD one) (NNS electrons)) (PP (IN in) (NP (DT the) (NN hydrogen) (NN atom))))) (. .)))