Table of Contents
- Quick Answer
- Atomic Mass vs. Mass Number
- The Five Natural Isotopes of Tungsten
- Wait, Is Tungsten Radioactive?
- What These Isotopes Are Actually Used For
- FAQ
Quick Answer
Tungsten has five naturally occurring isotopes: W-180, W-182, W-183, W-184, and W-186. Four are stable in every practical sense. The fifth, W-180, decays — so slowly that its half-life is about 1.8 quintillion years, roughly 130 million times the age of the universe.
| Isotope | Neutrons | Natural abundance | Status |
|---|---|---|---|
| W-180 | 106 | 0.12% | Alpha-radioactive, half-life ~1.8×10¹⁸ years |
| W-182 | 108 | 26.50% | Stable |
| W-183 | 109 | 14.31% | Stable, the only NMR-active tungsten isotope |
| W-184 | 110 | 30.64% | Stable, most abundant |
| W-186 | 112 | 28.43% | Stable |
That’s the whole natural inventory. Everything else with “tungsten” in its name — W-181, W-185, W-188, and a few dozen others — is synthetic, made in a reactor or accelerator and radioactive by design, not found in nature.
Atomic Mass vs. Mass Number
Tungsten’s atomic number is 74 — that’s fixed, it’s what makes the element tungsten and not something else. It’s the proton count. Neutrons are where isotopes differ: add or remove one and you get a different isotope of the same element, with the same chemistry but a different mass.
Mass number is protons plus neutrons, always a whole number — 180, 182, 183, and so on. Atomic mass, the 183.84 you see on the periodic table, is a weighted average of every naturally occurring isotope’s mass, scaled by how common each one is. It’s not the mass of any single atom you could point to. That’s why it lands between W-183 and W-184 instead of matching either one exactly — W-184 is the most abundant isotope, so it pulls the average closest to itself.
The Five Natural Isotopes of Tungsten
W-184 is the workhorse, making up nearly a third of any natural tungsten sample. W-182 and W-186 aren’t far behind, each contributing over a quarter. W-183 sits at 14.31% and would be unremarkable except for one property: it’s the only tungsten isotope with a nonzero nuclear spin (spin-1/2), which is the entire reason ¹⁸³W NMR spectroscopy exists as a technique.
Then there’s W-180, the odd one out. At 0.12% natural abundance, it’s rare enough that early isotope surveys sometimes missed it. It’s also the reason tungsten spent most of the 20th century listed as having five “stable” isotopes when it actually has four.
Wait, Is Tungsten Radioactive?
Mostly no — but W-180 broke that clean answer in 2004. Physicists using cryogenic calcium-tungstate crystals, originally built for a dark-matter detector, caught W-180 alpha-decaying into hafnium-176 for the first time. The measured half-life: (1.8 ± 0.2) × 10¹⁸ years, published in Physical Review C. For comparison, the universe is about 1.4 × 10¹⁰ years old. A gram of natural tungsten sitting on your desk is emitting a handful of alpha particles per year from its W-180 content — not zero, but not something a Geiger counter is built to notice either.
The other four isotopes aren’t proven stable, technically. Nuclear theory predicts some of them could eventually undergo alpha or double-beta decay, and physicists have set experimental lower limits on how long that would take — limits well beyond 10²⁰ years in most cases. Nobody has ever detected it happening. That’s the distinction between “stable” and “observationally stable,” and tungsten is a textbook case of it.
What These Isotopes Are Actually Used For
W-183 and NMR chemistry. Because it’s spin-1/2 and gives sharp resonance lines, ¹⁸³W NMR is the standard tool for characterizing polyoxometalates — large cluster molecules built from tungsten and oxygen that show up in catalysis and materials research. The signal is weak (low natural abundance plus a low resonance frequency make for a receptivity of just 0.059 relative to hydrogen), so these scans take patience. Chemists run them anyway, because nothing else shows the structure as clearly.
W-182 and fusion reactors. Tungsten is the leading candidate for lining the inside of a fusion reactor’s divertor — the part that takes the direct hit from superheated plasma — because of its extreme melting point. The problem is figuring out where the tungsten goes once the plasma starts eroding it. In 2020, researchers at Oak Ridge National Laboratory coated tungsten armor tiles inside the DIII-D tokamak with isotopically enriched W-182, then ran the reactor and tracked where that specific isotope ended up. Since enriched W-182 behaves identically to ordinary tungsten chemically, it worked as a tracer without changing anything about the plasma physics — the first experiment of its kind run inside an active fusion device.
W-188, made from natural tungsten, for cancer therapy. This one isn’t a natural isotope, but it starts from one: reactor operators irradiate W-186 with neutrons to produce W-188, which has a 69.4-day half-life and decays into rhenium-188. Hospitals use compact W-188/Re-188 generators — the same concept as the more familiar molybdenum-technetium generators — to draw off fresh Re-188 on demand for radionuclide therapy. Re-188 is a strong beta emitter, well suited to irradiating tumors directly, including liver cancers and synovial joint disease, without needing an on-site reactor.
FAQ
Is tungsten radioactive? Almost entirely no. Four of its five natural isotopes show no detected decay. The fifth, W-180, is radioactive but with a half-life so long — 1.8 quintillion years — that it has no practical or safety relevance.
How many isotopes does tungsten have? Five occur naturally: W-180, W-182, W-183, W-184, and W-186. Counting synthetic isotopes made in reactors and accelerators, isotope tables list several dozen more, all radioactive and all short-lived by comparison.
What’s the difference between atomic mass and mass number? Mass number is the whole-number total of protons and neutrons in one specific isotope, like 184 for W-184. Atomic mass, 183.84 for tungsten, is the natural-abundance-weighted average across all five isotopes — a statistical figure, not the mass of any actual atom.
Which tungsten isotope is used in NMR? W-183. It’s the only one with a nuclear spin that makes it detectable by nuclear magnetic resonance, and it’s the standard method for studying polyoxometalate structures in solution and solid state.
Why is tungsten used in fusion reactors specifically? Its melting point — over 3,400°C, the highest of any metal — lets it survive direct contact with plasma-facing components like the divertor without vaporizing, which is why isotope tracer studies like the ORNL work exist in the first place.
