( ( ( A molecule ) ( of ozone ) ) ( ( is ( formed ( by ( ( the ( chemical combination ) ) ( of ( three ( oxygen atoms ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (NN molecule)) (PP (IN of) (NP (NN ozone)))) (VP (VBZ is) (VP (VBN formed) (PP (IN by) (NP (NP (DT the) (JJ chemical) (NN combination)) (PP (IN of) (NP (CD three) (NN oxygen) (NNS atoms))))))) (. .)))
A molecule of ozone is formed by the chemical combination of three oxygen atoms.
(ROOT (S (NP (CD Three) (NN oxygen) (NNS ions)) (VP (VBP make) (PRT (RP up)) (NP (DT an) (NN ozone) (NN molecule))) (. .)))