( ( An ( oxygen atom ) ) ( ( ( gains ( two electrons ) ) ( to ( form ( the ( oxide ion ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (NN oxygen) (NN atom)) (VP (VBZ gains) (NP (CD two) (NNS electrons)) (S (VP (TO to) (VP (VB form) (NP (DT the) (NN oxide) (NN ion)))))) (. .)))
An oxygen atom gains two electrons to form the oxide 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)))))) (. .)))