( ( When ( ( one ( of ( these ions ) ) ) ( hits ( a ( hydrogen atom ) ) ) ) ) ( , ( it ( ( ( ( ( ( ( steals ( an electron ) ) back ) and ) ( becomes neutral ) ) , ) ( free ( to ( ( fly off ) ( in ( a ( straight line ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (CD one)) (PP (IN of) (NP (DT these) (NNS ions)))) (VP (VBZ hits) (NP (DT a) (NN hydrogen) (NN atom))))) (, ,) (NP (PRP it)) (VP (VP (VBZ steals) (NP (DT an) (NN electron)) (ADVP (RB back))) (CC and) (VP (VBZ becomes) (ADJP (JJ neutral))) (, ,) (S (ADJP (JJ free) (S (VP (TO to) (VP (VB fly) (PRT (RP off)) (PP (IN in) (NP (DT a) (JJ straight) (NN line))))))))) (. .)))
When one of these ions hits a hydrogen atom, it steals an electron back and becomes neutral, free to fly off in a straight line.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NNS electrons)) (VP (VBP move) (PP (IN from) (NP (CD one) (NN atom))) (PP (TO to) (NP (DT another)))))) (, ,) (NP (PRP they)) (VP (VBP become) (NP (NNS ions))) (. .)))