( ( ( ( An ( oxygen atom ) ) and ) ( two ( hydrogen atoms ) ) ) ( ( combine ( to ( ( form ( a ( water molecule ) ) ) ( -LRB- ( ( H ( 2 O ) ) -RRB- ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT An) (NN oxygen) (NN atom)) (CC and) (NP (CD two) (NN hydrogen) (NNS atoms))) (VP (VBP combine) (S (VP (TO to) (VP (VB form) (NP (DT a) (NN water) (NN molecule)) (PRN (-LRB- -LRB-) (NP (NNP H) (QP (CD 2) (CD O))) (-RRB- -RRB-)))))) (. .)))
An oxygen atom and two hydrogen atoms combine to form a water molecule (H 2 O).
(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.