Final answer:
The magnesium isotope with 13 neutrons is Magnesium-25 (25Mg). To find this, you add the atomic number of magnesium (12) to the number of neutrons (13) to get the mass number, which gives us Magnesium-25.
Step-by-step explanation:
The magnesium isotope with 13 neutrons can be determined by adding the number of protons (which is the same as the atomic number) to the number of neutrons. Since magnesium has an atomic number of 12, which means every magnesium atom has 12 protons, adding 13 neutrons gives us a mass number of 25 (12 protons + 13 neutrons = 25). Therefore, the magnesium isotope with 13 neutrons is Magnesium-25, which can be written as 25Mg or magnesium-25. The answer to the question is option 2) Magnesium-25.