A carbon isotope consists of 6 protons, 6 electrons and 8 neutrons, what is the mass number?

The is ##14##.

Since the mass number is simply the number of protons added with the number of neutrons, you would add ##6## to ##8## to get ##14##. Electrons are irrelevant in the calculation.

P.S. The mass number can also distinguish for , in this case a mass number of ##14## means that this carbon isotope is known as ##"Carbon"##-##14##.

The most common isotope of carbon is ##"Carbon"##-##12## and has ##6## protons and ##6## neutrons.