( ( ( 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 (JJ Monatomic) (NNS ions)) (VP (VBP form) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (JJ single) (NN atom)) (VP (VBZ gains) (CC or) (VBZ loses) (NP (NNS electrons)))))) (. .)))