How many protons, neutrons, and electrons are there in a neutral atom of 131i (iodine-131) ?

Atomic number od iodine is 53. That number show number of protons (in neutral atom number of electrons and protons are the same). So number of electrons are 53 and number of protons are 53.

Mass number of this isotope of iodine is 131. That number show number of neutrons + number of protons. So, number of neutrons:131-53=78.