( ( When ( ( an atom ) ( gains ( an electron ) ) ) ) ( it ( ( becomes ( a ( POSITIVE ion ) ) ) . ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT an) (NN atom)) (VP (VBZ gains) (NP (DT an) (NN electron))))) (NP (PRP it)) (VP (VBZ becomes) (NP (DT a) (JJ POSITIVE) (NN ion))) (. .)))
When an atom gains an electron it becomes a POSITIVE ion.
(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)))))) (. .)))