Uranium
Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-white metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons.
At a glance
- Atomic number
- 92
- Atomic mass
- 238.028913 u
- Phase
- Solid
- Density
- 19.1 g/cm³
- Melting point
- 1,405.3 K
- Boiling point
- 4,404 K
- First ionization energy
- 597.6 kJ/mol
- Electronegativity
- 1.38
- Electron affinity
- 50.94 kJ/mol
- Molar heat capacity
- 27.665 J/(mol·K)
- Period
- 7
- Appearance
- Unavailable
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Uranium’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
86 is the [Rn] core total across six shells.
Preparing the orbital view…
92 protons146 neutrons
Uranium-238: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, uranium is a solid.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
UNeutral atom · no charge
A neutral uranium atom. Removing its first electron takes 598 kJ/mol.
Uranium’s outer shell holds 2 electrons. Electron 3 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 1.7× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 7 | 598 | – | Measured |
| 2 | 7 | 1,119 | 1.9 | Measured |
| 3 | 6 | 1,910 | 1.7 | Measured |
| 4 | 6 | 3,541 | 1.9 | Measured |
| 5 | 6 | 4,438 | 1.3 | Calculated |
| 6 | 6 | 5,982 | 1.3 | Estimated |
| 7 | 6 | 8,587 | 1.4 | Calculated |
| 8 | 6 | 9,745 | 1.1 | Calculated |
| 9 | 6 | 11,192 | 1.1 | Calculated |
| 10 | 6 | 12,437 | 1.1 | Calculated |
| 11 | 6 | 15,245 | 1.2 | Calculated |
| 12 | 5 | 16,692 | 1.1 | Calculated |
| 13 | 5 | 20,262 | 1.2 | Calculated |
| 14 | 5 | 21,902 | 1.1 | Calculated |
| 15 | 5 | 31,165 | 1.4 | Calculated |
| 16 | 5 | 33,577 | 1.1 | Calculated |
| 17 | 5 | 36,182 | 1.1 | Calculated |
| 18 | 5 | 38,787 | 1.1 | Calculated |
| 19 | 5 | 41,585 | 1.1 | Calculated |
| 20 | 5 | 44,190 | 1.1 | Calculated |
| 21 | 5 | 47,953 | 1.1 | Calculated |
| 22 | 5 | 50,655 | 1.1 | Calculated |
| 23 | 5 | 53,742 | 1.1 | Calculated |
| 24 | 5 | 56,444 | 1.1 | Calculated |
| 25 | 5 | 70,434 | 1.2 | Calculated |
| 26 | 5 | 74,294 | 1.1 | Calculated |
| 27 | 5 | 77,188 | 1 | Calculated |
| 28 | 5 | 81,048 | 1.1 | Calculated |
| 29 | 5 | 89,731 | 1.1 | Calculated |
| 30 | 5 | 93,591 | 1 | Calculated |
| 31 | 5 | 103,239 | 1.1 | Calculated |
| 32 | 5 | 107,099 | 1 | Calculated |
| 33 | 4 | 116,747 | 1.1 | Calculated |
| 34 | 4 | 124,466 | 1.1 | Calculated |
| 35 | 4 | 132,185 | 1.1 | Calculated |
| 36 | 4 | 138,939 | 1.1 | Calculated |
| 37 | 4 | 146,658 | 1.1 | Calculated |
| 38 | 4 | 153,412 | 1 | Calculated |
| 39 | 4 | 161,131 | 1.1 | Calculated |
| 40 | 4 | 168,849 | 1 | Calculated |
| 41 | 4 | 176,568 | 1 | Calculated |
| 42 | 4 | 184,287 | 1 | Calculated |
| 43 | 4 | 192,006 | 1 | Calculated |
| 44 | 4 | 199,725 | 1 | Calculated |
| 45 | 4 | 206,479 | 1 | Calculated |
| 46 | 4 | 214,197 | 1 | Calculated |
| 47 | 4 | 248,739 | 1.2 | Calculated |
| 48 | 4 | 255,300 | 1 | Calculated |
| 49 | 4 | 262,247 | 1 | Calculated |
| 50 | 4 | 269,580 | 1 | Calculated |
| 51 | 4 | 276,623 | 1 | Calculated |
| 52 | 4 | 283,474 | 1 | Calculated |
| 53 | 4 | 296,499 | 1 | Calculated |
| 54 | 4 | 303,639 | 1 | Calculated |
| 55 | 4 | 311,455 | 1 | Calculated |
| 56 | 4 | 318,498 | 1 | Calculated |
| 57 | 4 | 347,540 | 1.1 | Calculated |
| 58 | 4 | 354,584 | 1 | Calculated |
| 59 | 4 | 362,109 | 1 | Calculated |
| 60 | 4 | 369,249 | 1 | Calculated |
| 61 | 4 | 406,589 | 1.1 | Calculated |
| 62 | 4 | 414,790 | 1 | Calculated |
| 63 | 4 | 435,438 | 1 | Calculated |
| 64 | 4 | 443,640 | 1 | Estimated |
| 65 | 3 | 713,316 | 1.6 | Calculated |
| 66 | 3 | 728,464 | 1 | Calculated |
| 67 | 3 | 745,832 | 1 | Calculated |
| 68 | 3 | 763,199 | 1 | Calculated |
| 69 | 3 | 780,566 | 1 | Calculated |
| 70 | 3 | 796,969 | 1 | Calculated |
| 71 | 3 | 834,598 | 1 | Calculated |
| 72 | 3 | 851,965 | 1 | Calculated |
| 73 | 3 | 871,263 | 1 | Calculated |
| 74 | 3 | 888,630 | 1 | Calculated |
| 75 | 3 | 937,837 | 1.1 | Calculated |
| 76 | 3 | 952,310 | 1 | Calculated |
| 77 | 3 | 968,713 | 1 | Calculated |
| 78 | 3 | 984,150 | 1 | Calculated |
| 79 | 3 | 1,100,898 | 1.1 | Calculated |
| 80 | 3 | 1,119,230 | 1 | Calculated |
| 81 | 3 | 1,156,859 | 1 | Calculated |
| 82 | 3 | 1,173,262 | 1 | Calculated |
| 83 | 2 | 2,437,219 | 2.1 | Calculated |
| 84 | 2 | 2,477,743 | 1 | Calculated |
| 85 | 2 | 2,523,091 | 1 | Calculated |
| 86 | 2 | 2,565,545 | 1 | Calculated |
| 87 | 2 | 2,996,834 | 1.2 | Calculated |
| 88 | 2 | 3,052,796 | 1 | Calculated |
| 89 | 2 | 3,126,125 | 1 | Calculated |
| 90 | 2 | 3,168,241 | 1 | Calculated |
| 91 | 1 | 12,501,595 | 3.9 | Calculated |
| 92 | 1 | 12,718,330 | 1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
Watching atoms decay
Uranium-238 halves so slowly that about half of what the Earth formed with is still here. Every atom that does decay sets off a relay of unstable atoms that only stops when it reaches lead.
Half of any sample of U-238 decays every 4.47 billion years, turning into Th-234.
Half-lives passed (1 half-life = 4.47 billion years)
No time passed · 0 half-lives · 100 of 100 atoms left
Each atom decays at a random moment, so no one can say which will go next, but about half of whatever is left goes in every half-life. The dashed curve is the prediction; the solid line is this sample. The new Th-234 atoms decay too, on their own schedule, which the chain below follows.
From U-238 to stable Pb-206: 14 steps
- U-238Uranium4.5 billion yearsαdecays by alpha
- Th-234Thorium24 daysβdecays by beta
- Pa-234mProtactinium1.2 minutesβdecays by beta
- U-234Uranium250,000 yearsαdecays by alpha
- Th-230Thorium75,000 yearsαdecays by alpha
- Ra-226Radium1,600 yearsαdecays by alpha
- Rn-222Radon3.8 daysαdecays by alpha
- Po-218Polonium3.1 minutesαdecays by alpha
- Pb-214Lead27 minutesβdecays by beta
- Bi-214Bismuth20 minutesβdecays by beta
- Po-214Polonium160 microsecondsαdecays by alpha
- Pb-210Lead22 yearsβdecays by beta
- Bi-210Bismuth5 daysβdecays by beta
- Po-210Polonium140 daysαdecays by alpha
- Pb-206Leadstable
Arrows show how each atom decays: α gives off an alpha particle (2 protons and 2 neutrons), β turns a neutron into a proton, γ gives off a gamma ray.
Half-lives and decay modes from the IAEA Live Chart of Nuclides (evaluated ENSDF data). The chain follows each atom’s most common decay; a few steps have rare side branches.
Splitting the atom
A uranium-235 nucleus that catches a slow neutron can split in two, releasing energy and 2 or 3 new neutrons. If those neutrons split more nuclei, the process feeds itself: a chain reaction. It’s what powers nuclear power stations.
One split, step by step
1. A slow neutron approaches
A neutron, with no electric charge, drifts towards a uranium-235 nucleus: 92 protons (red) and 143 neutrons (grey).
Proton NeutronDrag to rotate
Where the energy comes from: E = mc²
Weigh the neutron and the uranium atom, then weigh everything that comes out. Masses are in atomic mass units (u), about the mass of one proton.
- Neutron1.008665 u
- Uranium-235235.043928 u
Every particle counted and weighed. Now split it and weigh the pieces.
Split every atom in 1 kg of uranium-235 this way and just 0.79 g of mass disappears, less than a paper clip. That’s about 71 trillion joules, as much as burning about 3,000 tonnes of coal.
The chain reaction
Fire a neutron into the fuel. Add control rods to soak up neutrons, or change how much of the fuel is uranium-235, and try to keep the reaction steady, like a reactor operator.
Fire a neutron to start.
- Splits
- 0
- Neutrons flying
- 0
- Uranium-235 left
- 0
A toy model: a real reactor holds billions of billions of atoms, and water or graphite slows the neutrons so uranium-235 can catch them. Natural uranium is only 0.7% uranium-235; power-station fuel is enriched to a few percent. Barium-141 and krypton-92 are one common pair of fragments among many. Atomic masses from the 2020 Atomic Mass Evaluation (IAEA). Averaged over all the ways it can split, and counting energy the fragments give off later, a fission releases about 200 MeV. Coal taken as 24 MJ/kg.
How does it compare?
Density
19.1 g/cm³9th highest of 99
Melting point
1,405 K39th highest of 96
Boiling point
4,404 K13th highest of 97
Electronegativity
1.3840th lowest of 96
First ionization energy
598 kJ/mol34th lowest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- UraniumRoyal Society of Chemistry
- UraniumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database