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How big is a Neutron?

1.73 femtometer!

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Neutron

A neutron is a particle with no electric charge, found with protons in the nucleus of almost every atom. It is about 1.7 femtometers across, measured as twice its magnetic radius, which describes how its magnetism is spread out. A femtometer is a millionth of a billionth of a meter (10−15 meters).

Size

A neutron has no hard surface. It is made of three smaller particles called quarks, one up quark and two down quarks, held together by the strong force, the same force that binds neutrons and protons into a nucleus. The up quark has a positive charge and the down quarks have negative charges, and together they cancel out.

A proton's size is measured from how its electric charge is spread out, but that does not work for a neutron. Its positive and negative charges are spread out differently from each other, so the usual measure of charge size comes out as a negative number, which cannot be a width. Physicists use the neutron's magnetism instead.

A neutron acts as a tiny magnet. The Particle Data Group, which collects the world's particle measurements, gives the average radius of that magnetism as 0.864 femtometers, known to about 1%. Scattering experiments also show that neutrons and protons are about the same size.

Size comparisons

  • A neutron is about the same size as a proton. Measured the same way, by their magnetism, their radii differ by less than 2%.
  • A helium nucleus, made of two neutrons and two protons, is about 3.4 femtometers across, about twice the width of a single neutron.
  • If a neutron were enlarged to the size of a basketball, the nucleus of the most common kind of uranium, which holds 146 neutrons and 92 protons, would be about 1.6 meters (5 feet 4 inches) across.
  • About 60 billion neutrons lined up side by side would span the width of a human hair, about 0.1 millimeters.

How the neutron was found

In 1930, Walther Bothe and Herbert Becker fired alpha particles, which are helium nuclei, at the metal beryllium. It gave off a strong, penetrating radiation, which some physicists took to be high-energy radiation of the same kind as light. In 1932, James Chadwick, working at the Cavendish Laboratory in Cambridge, England, showed that it was made of neutral particles with about the same mass as a proton. Ernest Rutherford had suggested that such particles might exist in nuclei, and they were named neutrons.

Because a neutron has no charge, the positive charge of a nucleus does not push it away. So neutrons can reach and split even the heaviest nuclei, and Chadwick's discovery led the way to splitting uranium. He received the 1935 Nobel Prize in Physics for it.

A neutron on its own

Outside a nucleus, a neutron does not last long. On average it survives about 15 minutes (878 seconds) before it decays into a proton, an electron and a particle called an antineutrino. A neutron is slightly heavier than a proton, by about 0.14%.

Sources

De schaal van het Universum > Neutron
Proton
Protons are found within an atom's nucleus. They are thousands of times smaller than the atom itself. They have two up quarks and one down quark. Therefore, the proton's charge is +2/3+2/3-1/3 = +1.
Lengths shorter than this are not confirmed.
All the objects that are smaller than this are unmeasured. The sizes that they appear are only estimates. Some things, like quantum foam, are just hypothesized. They aren't fact.
Range of the Weak Force
The weak force is one of the four fundamental forces of nature, and is the weaker of the two nuclear forces. As distance increases, its strength decreases. At just 1 attometer, the weak force is so weak it is unmeasurable.
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!
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.
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.

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