( ( One ( oxygen atom ) ) ( ( ( combines ( with ( two ( hydrogen atoms ) ) ) ) ( to ( form ( a ( water molecule ) ) ) ) ) . ) )
(ROOT (S (NP (CD One) (NN oxygen) (NN atom)) (VP (VBZ combines) (PP (IN with) (NP (CD two) (NN hydrogen) (NNS atoms))) (S (VP (TO to) (VP (VB form) (NP (DT a) (NN water) (NN molecule)))))) (. .)))
One oxygen atom combines with two hydrogen atoms to form a water molecule.
(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.