( ( a -RRB- ) ( ( ( ( ( ( The ( basic structure ) ) ( of ( an atom ) ) ) ( is ( ( a ( small ( central nucleus ) ) ) ( composed ( of protons ) ) ) ) ) and ) ( neutrons ( surrounded ( by electrons ) ) ) ) . ) )
(ROOT (S (LST (LS a) (-RRB- -RRB-)) (S (NP (NP (DT The) (JJ basic) (NN structure)) (PP (IN of) (NP (DT an) (NN atom)))) (VP (VBZ is) (NP (NP (DT a) (JJ small) (JJ central) (NN nucleus)) (VP (VBN composed) (PP (IN of) (NP (NNS protons))))))) (CC and) (S (NP (NNS neutrons)) (VP (VBN surrounded) (PP (IN by) (NP (NNS electrons))))) (. .)))
a) The basic structure of an atom is a small central nucleus composed of protons and neutrons 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))))) (. .)))