Final answer:
The isotope of magnesium-25 (25Mg) has 12 protons, which is denoted by its atomic number 12 in the symbol 12Mg.
Step-by-step explanation:
The isotope of magnesium-25 (25Mg) has the same number of protons as any other isotope of magnesium, which is 12 protons. This is because the atomic number, which is noted as the subscript in front of the element symbol (in this case, 12Mg), signifies the number of protons in the nucleus of an atom of that element.
The mass number (25 in 25Mg) signifies the total number of protons and neutrons.
Since we know that all magnesium atoms have 12 protons, the magnesium-25 isotope must also have 12 protons, regardless of the number of neutrons or its mass number.