( ( ( ( Protons and ) neutrons ) ( in ( a ( central nucleus ) ) ) ) ( , ( ( surrounded ( by electrons ) ) . ) ) )
(ROOT (S (NP (NP (NNS Protons) (CC and) (NNS neutrons)) (PP (IN in) (NP (DT a) (JJ central) (NN nucleus)))) (, ,) (VP (VBN surrounded) (PP (IN by) (NP (NNS electrons)))) (. .)))
Protons and neutrons in a central nucleus, surrounded by electrons.
(ROOT (S (NP (NNS Protons) (CC and) (NNS neutrons)) (VP (VBP are) (VP (VBN located) (PP (IN in) (NP (DT the) (JJ central) (NN nucleus))))) (. .)))