Atomic Masses
Appendix: Selected Atomic and Nuclear Data
This appendix lists selected stable isotopes, naturally occurring radionuclides, and radionuclides commonly encountered in medicine, biology, environmental science, and introductory physics. It is not intended to be a complete table of nuclides.
The atomic masses shown are neutral-atom masses expressed in unified atomic mass units (u), also called daltons (Da). Natural abundances can vary slightly among samples. A blank half-life entry indicates that the isotope is considered stable for the purposes of this textbook.
| Atomic Number, Z | Element or Particle | Mass Number, A | Symbol | Atomic Mass (u or Da) | Natural Abundance or Common Application | Principal Decay Mode | Half-Life |
|---|---|---|---|---|---|---|---|
| 0 | Neutron | 1 | [latex]n[/latex] | 1.008 665 | Free neutron | [latex]\beta^-[/latex] | About 14.6 min |
| 1 | Hydrogen | 1 | [latex]{}^{1}\mathrm{H}[/latex] | 1.007 825 | About 99.99% | Stable | |
| 1 | Deuterium | 2 | [latex]{}^{2}\mathrm{H}[/latex] or D | 2.014 102 | About 0.015% | Stable | |
| 1 | Tritium | 3 | [latex]{}^{3}\mathrm{H}[/latex] or T | 3.016 049 | Biological and environmental tracer | [latex]\beta^-[/latex] | 12.32 y |
| 2 | Helium | 3 | [latex]{}^{3}\mathrm{He}[/latex] | 3.016 029 | Trace abundance; neutron detection | Stable | |
| 2 | Helium | 4 | [latex]{}^{4}\mathrm{He}[/latex] | 4.002 603 | More than 99.999% | Stable | |
| 5 | Boron | 10 | [latex]{}^{10}\mathrm{B}[/latex] | 10.012 937 | About 19.9%; neutron capture and boron neutron capture therapy research | Stable | |
| 5 | Boron | 11 | [latex]{}^{11}\mathrm{B}[/latex] | 11.009 305 | About 80.1% | Stable | |
| 6 | Carbon | 11 | [latex]{}^{11}\mathrm{C}[/latex] | 11.011 434 | PET imaging and research | [latex]\beta^+[/latex] | 20.33 min |
| 6 | Carbon | 12 | [latex]{}^{12}\mathrm{C}[/latex] | 12.000 000 | About 98.9% | Stable | |
| 6 | Carbon | 13 | [latex]{}^{13}\mathrm{C}[/latex] | 13.003 355 | About 1.1%; metabolic tracing and magnetic resonance spectroscopy | Stable | |
| 6 | Carbon | 14 | [latex]{}^{14}\mathrm{C}[/latex] | 14.003 242 | Radiocarbon dating and biological tracing | [latex]\beta^-[/latex] | 5730 y |
| 7 | Nitrogen | 13 | [latex]{}^{13}\mathrm{N}[/latex] | 13.005 739 | PET imaging, including myocardial perfusion | [latex]\beta^+[/latex] | 9.97 min |
| 7 | Nitrogen | 14 | [latex]{}^{14}\mathrm{N}[/latex] | 14.003 074 | About 99.6% | Stable | |
| 7 | Nitrogen | 15 | [latex]{}^{15}\mathrm{N}[/latex] | 15.000 109 | About 0.4%; stable-isotope tracer | Stable | |
| 8 | Oxygen | 15 | [latex]{}^{15}\mathrm{O}[/latex] | 15.003 066 | PET blood-flow and oxygen-metabolism studies | [latex]\beta^+[/latex] | 122.2 s |
| 8 | Oxygen | 16 | [latex]{}^{16}\mathrm{O}[/latex] | 15.994 915 | About 99.76% | Stable | |
| 8 | Oxygen | 18 | [latex]{}^{18}\mathrm{O}[/latex] | 17.999 160 | About 0.20%; stable tracer and production of [latex]{}^{18}\mathrm{F}[/latex] | Stable | |
| 9 | Fluorine | 18 | [latex]{}^{18}\mathrm{F}[/latex] | 18.000 938 | PET imaging, including fluorodeoxyglucose imaging | [latex]\beta^+[/latex] | 109.8 min |
| 9 | Fluorine | 19 | [latex]{}^{19}\mathrm{F}[/latex] | 18.998 403 | 100% | Stable | |
| 11 | Sodium | 22 | [latex]{}^{22}\mathrm{Na}[/latex] | 21.994 437 | Detector calibration and research | [latex]\beta^+[/latex], EC | 2.602 y |
| 11 | Sodium | 23 | [latex]{}^{23}\mathrm{Na}[/latex] | 22.989 770 | 100%; sodium magnetic resonance research | Stable | |
| 15 | Phosphorus | 31 | [latex]{}^{31}\mathrm{P}[/latex] | 30.973 762 | 100%; magnetic resonance spectroscopy | Stable | |
| 15 | Phosphorus | 32 | [latex]{}^{32}\mathrm{P}[/latex] | 31.973 907 | Biological tracer and selected therapies | [latex]\beta^-[/latex] | 14.27 d |
| 16 | Sulfur | 35 | [latex]{}^{35}\mathrm{S}[/latex] | 34.969 032 | Biochemical and molecular-biology tracer | [latex]\beta^-[/latex] | 87.4 d |
| 19 | Potassium | 39 | [latex]{}^{39}\mathrm{K}[/latex] | 38.963 707 | About 93.3% | Stable | |
| 19 | Potassium | 40 | [latex]{}^{40}\mathrm{K}[/latex] | 39.964 000 | About 0.0117%; naturally occurring radionuclide in the body | [latex]\beta^-[/latex], EC | [latex]1.25\times 10^{9}\ \mathrm{y}[/latex] |
| 20 | Calcium | 40 | [latex]{}^{40}\mathrm{Ca}[/latex] | 39.962 591 | About 96.9% | Stable | |
| 26 | Iron | 56 | [latex]{}^{56}\mathrm{Fe}[/latex] | 55.934 936 | About 91.8% | Stable | |
| 27 | Cobalt | 60 | [latex]{}^{60}\mathrm{Co}[/latex] | 59.933 822 | Radiation therapy, sterilization, and calibration | [latex]\beta^-[/latex] | 5.271 y |
| 29 | Copper | 64 | [latex]{}^{64}\mathrm{Cu}[/latex] | 63.929 765 | PET imaging and radiopharmaceutical therapy research | [latex]\beta^+[/latex], [latex]\beta^-[/latex], EC | 12.70 h |
| 31 | Gallium | 68 | [latex]{}^{68}\mathrm{Ga}[/latex] | 67.927 980 | PET imaging, including receptor-targeted imaging | [latex]\beta^+[/latex] | 67.7 min |
| 38 | Strontium | 89 | [latex]{}^{89}\mathrm{Sr}[/latex] | 88.907 451 | Palliation of pain from bone metastases | [latex]\beta^-[/latex] | 50.5 d |
| 38 | Strontium | 90 | [latex]{}^{90}\mathrm{Sr}[/latex] | 89.907 738 | Fission product and environmental contaminant | [latex]\beta^-[/latex] | 28.9 y |
| 39 | Yttrium | 90 | [latex]{}^{90}\mathrm{Y}[/latex] | 89.907 142 | Targeted radionuclide therapy and radioembolization | [latex]\beta^-[/latex] | 64.1 h |
| 40 | Zirconium | 89 | [latex]{}^{89}\mathrm{Zr}[/latex] | 88.908 889 | PET imaging with radiolabeled antibodies | [latex]\beta^+[/latex], EC | 78.4 h |
| 42 | Molybdenum | 99 | [latex]{}^{99}\mathrm{Mo}[/latex] | 98.907 708 | Parent radionuclide in technetium-99m generators | [latex]\beta^-[/latex] | 66.0 h |
| 43 | Technetium | 99m | [latex]{}^{99\mathrm{m}}\mathrm{Tc}[/latex] | Metastable nuclear state | Widely used diagnostic nuclear-medicine radionuclide | Isomeric transition | 6.01 h |
| 43 | Technetium | 99 | [latex]{}^{99}\mathrm{Tc}[/latex] | 98.906 250 | Daughter product of technetium-99m | [latex]\beta^-[/latex] | [latex]2.11\times 10^{5}\ \mathrm{y}[/latex] |
| 53 | Iodine | 123 | [latex]{}^{123}\mathrm{I}[/latex] | 122.905 590 | SPECT imaging, especially thyroid imaging | EC | 13.2 h |
| 53 | Iodine | 125 | [latex]{}^{125}\mathrm{I}[/latex] | 124.904 630 | Laboratory assays and brachytherapy | EC | 59.4 d |
| 53 | Iodine | 127 | [latex]{}^{127}\mathrm{I}[/latex] | 126.904 472 | 100% | Stable | |
| 53 | Iodine | 131 | [latex]{}^{131}\mathrm{I}[/latex] | 130.906 126 | Thyroid imaging and treatment | [latex]\beta^-[/latex] | 8.02 d |
| 54 | Xenon | 129 | [latex]{}^{129}\mathrm{Xe}[/latex] | 128.904 781 | About 26.4%; hyperpolarized-gas imaging research | Stable | |
| 54 | Xenon | 133 | [latex]{}^{133}\mathrm{Xe}[/latex] | 132.905 911 | Pulmonary ventilation and blood-flow studies | [latex]\beta^-[/latex] | 5.25 d |
| 55 | Cesium | 137 | [latex]{}^{137}\mathrm{Cs}[/latex] | 136.907 089 | Calibration, industrial sources, and environmental monitoring | [latex]\beta^-[/latex] | 30.05 y |
| 71 | Lutetium | 177 | [latex]{}^{177}\mathrm{Lu}[/latex] | 176.943 758 | Targeted radionuclide therapy | [latex]\beta^-[/latex] | 6.65 d |
| 77 | Iridium | 192 | [latex]{}^{192}\mathrm{Ir}[/latex] | 191.962 601 | High-dose-rate brachytherapy and industrial radiography | [latex]\beta^-[/latex], EC | 73.8 d |
| 82 | Lead | 208 | [latex]{}^{208}\mathrm{Pb}[/latex] | 207.976 652 | About 52.4% | Stable | |
| 84 | Polonium | 210 | [latex]{}^{210}\mathrm{Po}[/latex] | 209.982 874 | Naturally occurring radionuclide and environmental hazard | [latex]\alpha[/latex] | 138.4 d |
| 86 | Radon | 222 | [latex]{}^{222}\mathrm{Rn}[/latex] | 222.017 578 | Naturally occurring indoor-air hazard | [latex]\alpha[/latex] | 3.82 d |
| 88 | Radium | 223 | [latex]{}^{223}\mathrm{Ra}[/latex] | 223.018 502 | Targeted alpha therapy for selected bone metastases | [latex]\alpha[/latex] | 11.4 d |
| 88 | Radium | 226 | [latex]{}^{226}\mathrm{Ra}[/latex] | 226.025 410 | Naturally occurring radionuclide and parent of radon-222 | [latex]\alpha[/latex] | 1600 y |
| 89 | Actinium | 225 | [latex]{}^{225}\mathrm{Ac}[/latex] | 225.023 230 | Targeted alpha-therapy research and clinical applications | [latex]\alpha[/latex] | 9.92 d |
| 90 | Thorium | 232 | [latex]{}^{232}\mathrm{Th}[/latex] | 232.038 055 | Nearly 100% of naturally occurring thorium | [latex]\alpha[/latex] | [latex]1.40\times 10^{10}\ \mathrm{y}[/latex] |
| 92 | Uranium | 235 | [latex]{}^{235}\mathrm{U}[/latex] | 235.043 930 | About 0.72% of natural uranium; fissile | [latex]\alpha[/latex] | [latex]7.04\times 10^{8}\ \mathrm{y}[/latex] |
| 92 | Uranium | 238 | [latex]{}^{238}\mathrm{U}[/latex] | 238.050 788 | About 99.27% of natural uranium | [latex]\alpha[/latex] | [latex]4.47\times 10^{9}\ \mathrm{y}[/latex] |
Decay-Mode Abbreviations
- [latex]\alpha[/latex]: alpha decay
- [latex]\beta^-[/latex]: beta-minus decay
- [latex]\beta^+[/latex]: beta-plus decay or positron emission
- EC: electron capture
- Isomeric transition: transition from a metastable excited nuclear state to a lower-energy state, usually accompanied by gamma-ray emission or internal conversion
Notes
Stable isotopes: Some nuclides described as stable may be theoretically capable of decay but have no experimentally observed decay or have half-lives so long that they are treated as stable in introductory applications.
Natural abundance: Isotopic abundances may vary slightly depending on the origin and processing history of a sample. Values in this table are rounded representative values.
Atomic mass: Atomic masses include the electrons of a neutral atom. The unified atomic mass unit and the dalton are equivalent:
[latex]1\ \mathrm{u}=1\ \mathrm{Da}[/latex]
Medical use: A listed medical application does not imply that the unbound isotope itself is administered. Medical radionuclides are commonly incorporated into specific radiopharmaceuticals selected for a particular diagnostic or therapeutic purpose.
Finding the Most Up-to-Date Nuclear Data
The values in this appendix are suitable for introductory physics calculations and represent accepted values at the time this textbook was prepared. As scientific measurements improve, small revisions to atomic masses, isotopic abundances, half-lives, and decay data may occur.
Scientists, engineers, healthcare professionals, and researchers routinely consult authoritative online databases to obtain the most current values. Two of the most widely used resources are:
- National Institute of Standards and Technology (NIST): Atomic Weights and Isotopic Compositions
NIST Atomic Weights and Isotopic Compositions Database
Provides atomic masses, isotopic compositions, and related reference data maintained by the U.S. National Institute of Standards and Technology. - International Atomic Energy Agency (IAEA): LiveChart of Nuclides
IAEA LiveChart of Nuclides
Provides current information on nuclear structure, decay modes, half-lives, radiation energies, and other properties for thousands of known nuclides.
Students interested in nuclear medicine, medical physics, radiation therapy, radiochemistry, or nuclear engineering are encouraged to become familiar with these resources, as they are widely used in professional practice.
Data Sources
- International Atomic Energy Agency (IAEA), LiveChart of Nuclides.
- National Institute of Standards and Technology (NIST), Atomic Weights and Isotopic Compositions.
- Commission on Isotopic Abundances and Atomic Weights (CIAAW), International Union of Pure and Applied Chemistry (IUPAC).
