( ( a molecule ) ( ( has ( ( ( 2 ( hydrogen atoms ) ) ( but only ) ) ( 1 oxygen ) ) ) . ) )
(ROOT (S (NP (DT a) (NN molecule)) (VP (VBZ has) (NP (NP (CD 2) (NN hydrogen) (NNS atoms)) (CONJP (CC but) (RB only)) (NP (CD 1) (NN oxygen)))) (. .)))
a molecule has 2 hydrogen atoms but only 1 oxygen.
(ROOT (S (NP (NP (CD 2) (NN hydrogen)) (, ,) (NP (CD 1) (NN oxygen) (NNS atoms))) (VP (VBP combine) (S (VP (TO to) (VP (VB make) (PRT (RP up)) (NP (NP (DT a) (NN molecule)) (PP (IN of) (NP (NN water)))))))) (. .)))
2 hydrogen, 1 oxygen atoms combine to make up a molecule of water.