J. J.ThompsonOctetRuleProtonAtomicnumberIsotopesCompoundsElectronGroundstateExcitedStateCenterof theatomErnestRutherfordOutsidenucleusHigherEnergyLevelsElectronCloudModelJohnDaltonNielsBohrLowerEnergyLevelsInsidenucleusAristotleNegativeValenceuNeutronsMassnumberDemocritus