Which particle of an atom has the least mass? A. proton B. nucleus C. neutron D. electron

D. electron electrons have the smallest mass in an atom

We can identify the experimental masses of the particles given to determine which of the following has the least mass. In this case, the mass of the proton is equal to 1.6726219 * 10-27 kilograms; nucleus is equal to 1.674927471 x 10-27 kilograms; electron has a mass of 9.10938356 * 10-31 kilograms. A nucleus' mass is the sum of the mass of neutrons and protons present inside. Hence the answer is D. electron.