Shells (electrons) - located in the cloud nucleus- located in the center energy level - is determined by the electron count ( level 1 - 2 electrons level 2 - 8 electrons level 3- 18 electrons)
Roughly 96 percent of the mass of the human body is made up of just four elements: oxygen, carbon, hydrogen and nitrogen, with a lot of that in the form of water. The remaining 4 percent is a sparse sampling of the periodic table of elements