Final answer:
The most common isotope of arsenic has 42 neutrons, calculated by subtracting the atomic number (33) from the atomic mass number (75).
Step-by-step explanation:
The number of neutrons in arsenic (As) can be determined by looking at its atomic mass and its atomic number. The atomic number of arsenic is 33, which means it has 33 protons. The most common isotope of arsenic is arsenic-75, which has an atomic mass of approximately 74.92160 amu (atomic mass units). The number of neutrons is found by subtracting the atomic number from the atomic mass number. So, for the most common isotope, the calculation would be 75 (rounded atomic mass number) - 33 (number of protons) = 42 neutrons.