3—3 Practice Problems (continued) 15. Write the chemical symbol for the ion with 21. How many protons, neutrons, and 29 protons and 27 electrons, electrons are present in the n4 ion? 16. How many protons, neutrons, and 22. Write the complete chemical symbol for electrons are present in the Ni2 ion? the ion with 84 protons, 125 neutrons, and ...