JohnDaltonNeutronsValenceCenterof theAtomElectronCloudModelElectronErnestRutherfordAristotleInsideNucleusCompoundsDemocritusExcitedStateIsotopesProtonOctetRuleNielsBohrOutsideNucleusNegativeLowerEnergyLevelsAtomicJ.J.ThompsonSpectrumGroundStateMassNumber