( ( When ( ( a ( hydrogen atom ) ) ( gains ( an electron ) ) ) ) ( , ( it ( ( becomes ( a ( negative ion ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT a) (NN hydrogen) (NN atom)) (VP (VBZ gains) (NP (DT an) (NN electron))))) (, ,) (NP (PRP it)) (VP (VBZ becomes) (NP (DT a) (JJ negative) (NN ion))) (. .)))
When a hydrogen atom gains an electron, it becomes a negative ion.
(ROOT (S (NP (NP (NNS Atoms)) (SBAR (WHNP (WDT that)) (S (VP (VBP have) (VP (VP (VBN gained) (NP (NNS electrons))) (CC and) (VP (VBN become) (S (VP (ADVP (RB negatively)) (VBN charged))))))))) (VP (VBP are) (VP (VBN called) (NP (JJ negative) (NNS ions)))) (. .)))