( ( ( an atom ) ( that ( gains ( an electron ) ) ) ) ( ( forms ( ( ( a ( ( negatively charged ) ion ) ) , ) ( called ( an anion ) ) ) ) . ) )
(ROOT (S (NP (NP (DT an) (NN atom)) (SBAR (WHNP (WDT that)) (S (VP (VBZ gains) (NP (DT an) (NN electron)))))) (VP (VBZ forms) (NP (NP (DT a) (ADJP (ADVP (RB negatively)) (VBN charged)) (NN ion)) (, ,) (VP (VBN called) (NP (DT an) (NN anion))))) (. .)))
an atom that gains an electron forms a negatively charged ion, called an anion.
(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)))) (. .)))