Ilungu ngalinye linohlobo lwe-atom olunye kuphela. Inani lee-protons ezikwi-atom libizwa ngokuba yi-atomic number okanye yinombolo ye-atom, umzekelo, zonke ii-atoms ezinee-protons ezi-6 zezelungu lekhemikhali eliyi-carbon, kwaye zonke ii-atoms ezinee-protons ezingama-92 zona zezelungu ekuthiwa yi-uranium.