Atomic weights Aufbau’ principle

Dalton s atoms were not perceived as possessing structure, but the discovery of electrons, and the distinctive phenomenon of radioactivity, inevitably generated interest in the way atoms were put together. Clearly, this had a bearing on the periodic table as any proposed structure must explain atomic weights.24"28 The Aufbau Principle, that each element possessed one more proton in the nucleus and one more electron in the outer shell than the preceding element, effectively systematized the periodic table29-30... 

---