Chapter 6 The Chemistry of Life Atoms and Their Interactions Elements Carbon, hydrogen, oxygen, and nitrogen make up more than 96% of the mass of a human body. Atoms All nuclei contain: positively charged particles called protons (p+) particles with no charge called neutrons (n0). Electron cloud The region of space surrounding the nucleus contains extremely small, negatively charged particles called electrons (e-) This region of space is referred to as an electron cloud. Energy levels The first energy level can hold only two electrons. The second level can hold a maximum of eight electrons. The third level can hold up to 18 electrons. How many electrons are in fluorine?s 2nd energy level? How many protons are in an atom of fluorine? Isotopes