Answer:
Argon.
Explanation:
Argon is the element having 18 protons and 22 neutrons present in its nucleus. so its atomic number is 18 and mass number is 40 if we added both number of protons and neurons. Argon belongs to noble family due to completion of outermost shell and non reactive nature. It is the third most abundant gas about 0.934% present on the earth after nitrogen and oxygen.
