How big is Chlorine Nucleus? | Scale of the Universe

A chlorine nucleus is the center of a chlorine atom. It holds 17 protons and, in the two kinds that make up natural chlorine, either 18 or 20 neutrons. The nucleus of the more common kind, chlorine-35, is about 6.7 femtometers across, measured as twice its charge radius. 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, the average radius of its electric charge. For chlorine-35, a table of nuclear radii kept by the International Atomic Energy Agency (IAEA) gives 3.365 femtometers. A more precise measurement reported in 2026, using atoms in which a heavier particle called a muon takes an electron's place, found 3.333 femtometers. Both values make the nucleus about 6.7 femtometers across.

Atoms of the same element with different numbers of neutrons are called isotopes. Chlorine-37 has two more neutrons than chlorine-35, but its nucleus is less than 1% wider. About three-quarters of the chlorine on Earth is chlorine-35.

Like most nuclei with more than 15 protons, chlorine's nucleus has more neutrons than protons. Protons push one another apart with their electric charge, and the extra neutrons help the strong nuclear force hold the nucleus together.

Size comparisons

A chlorine-35 nucleus is about four times as wide as a single proton, which is about 1.7 femtometers across.
It is about twice as wide as a helium nucleus, which is about 3.4 femtometers across.
A uranium nucleus holds 6.8 times as many protons and neutrons, but it is less than twice as wide.
In a molecule of chlorine gas, the nuclei of the two atoms are 0.199 nanometers apart, about 30,000 times the width of either nucleus. A nanometer is a billionth of a meter.
If those two nuclei were enlarged to the size of basketballs, they would be about 7 kilometers (4.4 miles) apart.

Catching neutrinos from the Sun

Chlorine-37 nuclei were used to detect neutrinos from the Sun, particles that pass through almost any matter. Very rarely, a neutrino striking a chlorine-37 nucleus turns one of its neutrons into a proton. The nucleus becomes argon-37, which is radioactive and has a half-life of 35 days, the time it takes half of it to decay.

In the 1960s, Raymond Davis Jr. placed a tank of 615 metric tons (678 US tons) of tetrachloroethylene, a common cleaning fluid, in a gold mine in South Dakota. It held about 2 × 10³⁰ chlorine-37 atoms, a 2 followed by 30 zeros. By 1994, he had extracted about 2,000 argon atoms, fewer than expected. Davis shared the 2002 Nobel Prize in Physics for this work.