( ( ( the periods ) ( of ( the ( periodic table ) ) ) ) ( ( relate ( to ( ( the number ) ( of ( ( electron ( shells ( or ( energy levels ) ) ) ) ( of ( constituent atoms ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT the) (NNS periods)) (PP (IN of) (NP (DT the) (JJ periodic) (NN table)))) (VP (VBP relate) (PP (TO to) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NN electron) (NNS shells) (CC or) (NN energy) (NNS levels)) (PP (IN of) (NP (JJ constituent) (NNS atoms)))))))) (. .)))
the periods of the periodic table relate to the number of electron shells or energy levels of constituent atoms.
(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)))))) (. .)))