( ( In ( the ( first ( successful ( atomic model ) ) ) ) ) ( , ( ( ( ( Bohr 's ) ( electron ( energy levels ) ) ) ( -LRB- ( ( which ( ( at first ) ( appeared ( very arbitrary ) ) ) ) -RRB- ) ) ) ( ( could ( be ( better ( understood ( if ( ( the electron ) ( was ( considered ( ( a ( `` ( wave '' ) ) ) ( orbiting ( the nucleus ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (DT the) (JJ first) (JJ successful) (JJ atomic) (NN model))) (, ,) (NP (NP (NP (NNP Bohr) (POS 's)) (JJ electron) (NN energy) (NNS levels)) (PRN (-LRB- -LRB-) (SBAR (WHNP (WDT which)) (S (ADVP (IN at) (JJ first)) (VP (VBD appeared) (ADJP (RB very) (JJ arbitrary))))) (-RRB- -RRB-))) (VP (MD could) (VP (VB be) (VP (ADVP (RBR better)) (VBN understood) (SBAR (IN if) (S (NP (DT the) (NN electron)) (VP (VBD was) (VP (VBN considered) (NP (NP (DT a) (`` ``) (NN wave) ('' '')) (VP (VBG orbiting) (NP (DT the) (NN nucleus))))))))))) (. .)))
In the first successful atomic model, Bohr's electron energy levels (which at first appeared very arbitrary) could be better understood if the electron was considered a &quot;wave&quot; orbiting the nucleus.
(ROOT (S (NP (NP (NNS Electrons)) (PP (IN in) (NP (NP (JJ successive) (NNS atoms)) (PP (IN on) (NP (DT the) (JJ periodic) (NN table)))))) (VP (VBP tend) (S (VP (TO to) (VP (VB fill) (NP (JJ low-energy) (NNS orbitals)) (ADVP (RB first)))))) (. .)))