What element has a mass number of 40 and 20 protons? Chemistry Matter Mass Number 1 Answer anor277 Sep 15, 2016 #""^40Ca#. Explanation: If there are 20 protons, then #Z=20#, and thus the element is calcium. There are a further 20 neutrons in the nucleus, but this just determines the isotope. The #""^40Ca# isotope has approx. 97% natural abundance. Answer link Related questions How do you find mass number of an isotope? What does the difference between the mass number and the atomic number tell us? Why is mass number a decimal? What type of radioactive decay produces no change in mass number? How is the mass number written? What is the nucleon number? What is mass number equal to? What does the mass number tell us? How is mass number related to isotopes? How is the atomic mass number changed by gamma decay? See all questions in Mass Number Impact of this question 8518 views around the world You can reuse this answer Creative Commons License