the mass number of an atom equals; how many neutrons are in an atom of tin? round your answer to the nearest whole number.; choose the species with 15 protons and 18 electrons.; how many protons and electrons are present in a lead atom?; co is the symbol for; an atom with 45 protons has a mass number of 100. how many neutrons does the atom contain?; how many protons are in an atom of tin?; how many neutrons does an atom of tungsten have? round your answer to the nearest whole number.