Try Shutterstock and Get 10 Free Images
Atomic number and mass number of ordinary atoms, using helium as an example. The atomic number (Z) is also the number of protons (np). The Mass number (A) is the total number of neutrons and protons.
Electron shells of the first ordinary elements of the periodic table. In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atomic nucleus.