( ( When ( ( ( 2 atoms ) ( of hydrogen ) ) ( ( chemically bond ) ( to ( ( 1 atom ) ( of oxygen ) ) ) ) ) ) ( , ( ( ( a molecule ) ( of water ) ) ( ( is formed ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (CD 2) (NNS atoms)) (PP (IN of) (NP (NN hydrogen)))) (VP (VBD chemically) (NP (NN bond)) (PP (TO to) (NP (NP (CD 1) (NN atom)) (PP (IN of) (NP (NN oxygen)))))))) (, ,) (NP (NP (DT a) (NN molecule)) (PP (IN of) (NP (NN water)))) (VP (VBZ is) (VP (VBN formed))) (. .)))
When 2 atoms of hydrogen chemically bond to 1 atom of oxygen, a molecule of water is formed.
(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.