The element lithium has two stable isotopes, li-6 and li-7. how do the atomic structures of these two isotopes compare? a) both have 3 protons and 3 electrons, but li-7 has 4 neutrons. b) both have 3 protons and 4 neutrons, but li-7 has 4 electrons. c) li-6 has 3 protons and 3 electrons; li-7 has 3 protons and 4 electrons. d) li-6 has 3 protons, 3 neutrons, 6 electrons, while li-7 has 3 protons, 3 neutrons, and 7 electrons.