( ( ( A combination ) ( of ( ( protons and ) neutrons ) ) ) ( ( combine ( to ( form ( ( the nucleus ) ( of ( an atom ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (NN combination)) (PP (IN of) (NP (NNS protons) (CC and) (NNS neutrons)))) (VP (VBP combine) (S (VP (TO to) (VP (VB form) (NP (NP (DT the) (NN nucleus)) (PP (IN of) (NP (DT an) (NN atom)))))))) (. .)))
A combination of protons and neutrons combine to form the nucleus of an atom.
(ROOT (S (NP (NP (DT The) (NN atom)) (PP (IN in) (NP (DT a) (NN neutron)))) (VP (VBZ is) (VP (VBN found) (PP (IN in) (NP (DT the) (DT the) (NN nucleus))))) (. .)))