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How big is a Uranium Nucleus?

12 femtometres!

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Uranium Nucleus

A uranium nucleus is the center of a uranium atom. It holds 92 protons and, in uranium-238, the most common kind, 146 neutrons. It is about 12 femtometers across, measured as twice its charge radius, the average radius of its electric charge. A femtometer is a millionth of a billionth of a meter (10−15 meters).

Size

A nucleus has no hard edge, so its size is given as a charge radius. For uranium-238, a table of nuclear radii kept by the International Atomic Energy Agency (IAEA) gives 5.857 femtometers, known to better than 0.1%. That makes the nucleus 11.7 femtometers across. Uranium-235, which has three fewer neutrons, is very slightly smaller, 11.67 femtometers across.

A uranium nucleus has far more neutrons than protons. Protons push one another apart with their electric charge, while all the protons and neutrons pull on their nearest neighbors through the strong nuclear force. In a heavy nucleus, the extra neutrons are needed to keep the electric push from breaking it apart.

Size comparisons

  • A uranium-238 nucleus is about 7 times as wide as a single proton, which is about 1.7 femtometers across, even though it holds 238 protons and neutrons.
  • It is about 3.5 times as wide as a helium nucleus, which is about 3.4 femtometers across, but it holds about 60 times as many particles.
  • If a uranium nucleus were enlarged to the size of a basketball, a proton on the same scale would be about 3.4 centimeters (1.4 inches) across, about half the width of a tennis ball.
  • A hydrogen atom, about 0.1 nanometers across, is about 9,000 times as wide as a uranium nucleus. A nanometer is a billionth of a meter.

A very slow decay

Uranium-238 is radioactive, but it decays extremely slowly. Its half-life, the time it takes half of a sample to decay, is 4.47 billion years. Earth is about 4.54 billion years old, so about half of the uranium-238 that Earth formed with is still here.

Uranium-238 decays by throwing out an alpha particle, which is a helium nucleus. A chain of further decays follows, ending in lead-206. Geologists use this chain, and a similar one that starts from uranium-235, to measure the ages of rocks and of Earth itself.

Splitting uranium

Uranium-235 makes up about 0.72% of natural uranium. Its nuclei split apart easily when struck by a neutron. Each split releases energy and more neutrons, which can go on to split other nuclei in a chain reaction. Nuclear power plants in the United States control this reaction to make heat for electricity, using fuel enriched to 3–5% uranium-235.

Sources

Scale of the Universe > Uranium Nucleus
Chlorine Nucleus
A chlorine nucleus has 17 protons and anywhere from 11 to 34 neutrons. However, the majority of these isotopes will decay within minutes, if not seconds! The only stable isotopoes are chlorine-35 (18 neutrons) and chlorine-37 (20 neutrons). Even numbers of neutrons (especially magic numbers 2, 8, 20, 28, 50, 82, and 126) tend to be more stable than odd ones.
Electron (Classical)
Electrons are so small that their size can't accurately be measured. The size of an electron varies greatly depending on how it's measured, whether it's based on the quantum model or, in this case, the classical model.
Helium Nucleus
The helium nucleus is thousands of times smaller than the atom, like a marble in a football field. The only reason matter feels solid is because atoms repel. If atoms didn't repel, everything would fall through each other!
Gamma Ray Wavelength
Gamma rays have a very high frequency. They can come from radioactive decay. Oh, and by the way, it's not actually light blue as it appears to the left.
Smallest Thing Visible to an Electron Microscope
Electron microscopes allow us to see very small things (like atoms) that would otherwise be invisible because they are smaller than the wavelength of visible light.
Helium Atom
Hydrogen atoms are larger than helium atoms because more protons in an atom's nucleus pull the electrons in closer. However, additional shells increase the size of the atoms the most. Cesium, the largest atom, has the most shells with the fewest electrons. (Francium has not been measured.)

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