The atomic number of carbon-13 is thirteen. The atomic mass of carbon-13 is thirteen. If different atoms have the same number of protons but different numbers of neutrons they are called ____________ . Substances composed of atoms that all have the same number of protons are called ____________ . Atoms that have gained or lost electrons and are no longer electrically neutral are called ____________ . The nucleus of an atom always contains one or more ____________ . The orbitals of an atom contain negatively charged particles called ____________ .