An isotope of iodine-131 has how many protons?

• A. 78 Protons
• B. 131 Protons
• C. 82 Protons
• D. 53 Protons

Answer: (D)

Protons define the element, and isotopes share the same number of protons. Iodine has 53 protons (its atomic number), so every isotope of iodine has 53 protons. The notation iodine-131 means the total number of nucleons (protons plus neutrons) is 131, so neutrons are 131 minus 53, which is 78. Therefore, the number of protons is 53. The other numbers correspond to neutrons or the total nucleons, not to the proton count.