( ( For example ) ( , ( ( ( ( ( carbon ( ( needs ( four electrons ) ) ( to ( complete ( its ( valence shell ) ) ) ) ) ) , ) so ) ( it ( forms ( four bonds ) ) ) ) . ) ) )
(ROOT (S (PP (IN For) (NP (NN example))) (, ,) (S (NP (NN carbon)) (VP (VBZ needs) (NP (CD four) (NNS electrons)) (S (VP (TO to) (VP (VB complete) (NP (PRP$ its) (JJ valence) (NN shell))))))) (, ,) (IN so) (S (NP (PRP it)) (VP (VBZ forms) (NP (CD four) (NNS bonds)))) (. .)))
For example, carbon needs four electrons to complete its valence shell, so it forms four bonds.
(ROOT (S (NP (NNP Carbon)) (VP (VBZ has) (NP (CD four) (JJ valence) (NNS electrons))) (. .)))