Concept:Atomic number equals mass number only when neutrons are absent from the nucleus.
Explanation:Atomic number is the number of protons.
Mass number is the number of protons plus neutrons.
So, mass number = atomic number + number of neutrons.
For mass number to equal atomic number, the number of neutrons must be zero.
Ordinary hydrogen has atomic number
1 and mass number
1.
Its nucleus contains only one proton and no neutrons.
Hence, this hydrogen isotope has equal atomic and mass numbers because it does not contain neutrons.
Answer:C. does not contain neutrons