Why does uranium have an atomic weight when it has no stable isotope?
For most radioactive elements, this page shows the atomic weight in brackets: the mass number of the longest-lived isotope, rather than a measured average. Technetium, promethium and everything past bismuth are handled that way.
Uranium is an exception. IUPAC assigns it a conventional standard atomic weight of 238.02891 u, a precise measured value like any stable element’s.
The reason is that an atomic weight is only meaningful if the isotopic mixture is consistent enough to average. For most radioactive elements it is not — they exist in trace quantities whose composition depends entirely on how they were produced.
Uranium is different because it is primordial. Both of its main isotopes have half-lives comparable to the age of the Earth: uranium-238 at about 4.47 billion years, uranium-235 at about 704 million years. Both have survived since the planet formed, and both are thoroughly mixed through the crust.
The resulting composition is remarkably uniform: 99.274% uranium-238, 0.720% uranium-235, and 0.0055% uranium-234 in essentially every natural sample. That consistency is what makes an average meaningful.
Thorium and protactinium are handled the same way, for the same reason.
The uniformity is precise enough that departures from it are diagnostic. A sample with less than 0.72% uranium-235 has had material removed, which is how depleted uranium is identified. And the one natural exception on Earth — uranium at Oklo measurably depleted below the standard figure — was what revealed a natural fission reactor had operated there.
Why does the difference between U-235 and U-238 matter so much?
Three neutrons separate the two isotopes. Chemically they are identical. Their nuclear behaviour is not, and the difference decides whether uranium is a fuel or just a heavy metal.
Uranium-235 is fissile. Absorbing a slow neutron leaves it in an excited state above its fission barrier, and it splits, releasing energy and two or three further neutrons. Those neutrons can trigger further fissions — a chain reaction.
Uranium-238 is not fissile by slow neutrons. Absorbing one leaves it below the barrier, and instead of splitting it decays to plutonium-239. It is fertile rather than fissile.
The origin of the difference is nuclear pairing. Uranium-235 has an odd number of neutrons, so an added neutron pairs up and releases pairing energy, which supplies the extra push over the fission barrier. Uranium-238 has an even number, no pairing bonus is available, and the barrier is not cleared.
Since only 0.72% of natural uranium is the fissile isotope, most reactor designs need it concentrated to 3–5%. Because the isotopes are chemically identical, separation must exploit the 1.3% mass difference — done industrially by converting uranium to the gas uranium hexafluoride and spinning it in cascades of centrifuges.
That process is the hinge of nuclear proliferation policy. The same centrifuges that reach 5% for power reactors reach 90% for weapons, given more stages and more time. The technology is not separable by purpose.
A minority of reactor designs avoid enrichment entirely. The CANDU heavy-water design uses natural uranium, made possible by deuterium’s low neutron absorption, which preserves enough neutrons to sustain the chain reaction at 0.72%.
How do you read uranium’s electron configuration?
Uranium’s ground state is [Rn] 5f³ 6d¹ 7s²: a radon core, three electrons in 5f, one in 6d, two in 7s. Electrons per shell: 2, 8, 18, 32, 21, 9, 2.
This is one of the 21 documented exceptions to the Madelung filling order, which predicts [Rn] 5f⁴ 7s². One electron sits in 6d instead of 5f, because at this point in the actinide series the 5f and 6d levels are so close in energy that electron repulsion decides the arrangement rather than the ordering rule.
The near-degeneracy of 5f and 6d is what makes early actinide chemistry so much richer than lanthanide chemistry. In the lanthanides, 4f electrons are buried and take little part in bonding, so nearly every lanthanide is +3 and they are notoriously hard to separate. Uranium’s 5f electrons are more exposed and chemically available, and uranium shows oxidation states from +3 to +6.
The dominant state is +6, as the uranyl ion UO₂²⁺, which is what makes uranium mobile in groundwater and gives uranium compounds their yellow-green colours. Uranium glass, coloured with uranium oxide, fluoresces bright green under ultraviolet light for the same reason.
Uranium’s density is about 19,050 kg/m³, close to gold’s and nearly twice lead’s.
What did uranium look like before anyone knew what it did?
Uranium was identified by Martin Klaproth in 1789, in pitchblende, and named after the planet Uranus discovered eight years earlier. For the next century it was an obscure industrial colourant — used to give glass and ceramic glazes yellow and orange tints, valued for the effect and nothing else.
Its significance arrived by accident. In 1896 Henri Becquerel left uranium salts on a wrapped photographic plate in a drawer, expecting that sunlight would be needed to produce any effect. The plate was exposed anyway. Radioactivity had been discovered, and it was discovered because of uranium.
Marie and Pierre Curie then worked through tonnes of pitchblende residues, tracking activity that uranium alone could not account for, and isolated polonium and radium. The field of nuclear physics grew from that pitchblende.
In 1938 Otto Hahn and Fritz Strassmann found barium among the products of neutron-bombarded uranium — a result that made no sense until Lise Meitner and Otto Frisch interpreted it as the nucleus splitting. Within seven years that interpretation had produced both nuclear reactors and nuclear weapons.
Uranium glassware from the 1930s is still collected, and still mildly radioactive.
How much energy is in uranium?
The numbers are what make nuclear power a distinct category rather than another fuel.
Complete fission of one kilogram of uranium-235 releases roughly the energy of burning two to three million kilograms of coal. In practice reactors do not achieve complete fission, but even at typical burn-up the ratio remains in the tens of thousands.
The concrete version: a standard fuel pellet is about a centimetre across, weighs around seven grams, and delivers energy comparable to roughly a tonne of coal. A typical large reactor is refuelled with a truckload of fuel assemblies every eighteen months, where a coal plant of the same output consumes a train of coal every day or two.
This density is the source of both the appeal and the difficulty. The waste is correspondingly small in volume — all the spent fuel ever produced by the United States civilian programme would cover a single football field to a depth of a few metres — but it is intensely radioactive and remains hazardous for far longer than any human institution has existed. Long-lived actinides and fission products drive the requirement for isolation on the order of a hundred thousand years.
No country has yet opened a permanent deep geological repository for civilian spent fuel, although Finland’s Onkalo facility is the furthest advanced.