Table 1: Atomic Masses of Selected Isotopes hydrogen H 1.007825 magnesium-24 Mg 23.985042 neutron no 1.008665 silicon-28 Si 27.976927 deuterium H (or D) 2.014102 potassium-40 K 39.963998 tritium H (or T) 3.016049 calcium-40 Ca 39.962591 helium-3 He 3.016029 cobalt-60 Co 59.993817 helium-4 He 4.002603 cesium-140 Cs 139.905439 boron-12 B 12.003510 barium-141 Ba 140.914412 carbon-12 C 12.000000 lead-208 Pb 207.976652 nitrogen-16 N 16.006102 radon-222 Rn 222.017578 oxygen-16 O 15.994915 radium-226 Ra 226.025410 neon-20 Ne 19.992440 thorium-230 Th 230.033134 neon-22 Ne 21.991385 thorium-234 Th 234.043601 sodium-22 Na 21.994436 uranium-235 U 235.043930 sodium-23 Na 22.989769 uranium-238 C 238.050788