if an atom has 13 electrons how many protons does it have; an atom has 15 total electrons. how many electrons does it have in its outer shell?; if an atom has 12 electrons how many shells will it have; an atom with 20 total electrons.; how many shells does hydrogen which has one electron have; if an atom has 18 electrons, which of the following represent the electron configuration?; what does the atomic number tell us on the periodic table?; which of the following atoms is most likely to bond with another atom?