To calculate the atomic mass<span> of a single </span>atom<span> of an </span>element<span>, add up the </span>mass<span> of protons and neutrons. Example: Find the </span>atomic mass<span> of an isotope of carbon that has 7 neutrons. You can see from the periodic table that carbon has an </span>atomic number<span> of 6, which is its </span>number<span> of protons.</span>