The atomic symbol superscript 40 subscript 19 upper k. represents potassium-40 (k-40), a radioactive isotope that has 19 protons and 21 neutrons. which atomic symbol could represent an isotope that results from the decay of k-40? superscript 40 subscript 18 upper a r. superscript 40 subscript 18 upper k superscript 42 subscript 20 upper c a superscript 42 subscript 20 upper k.