If a hydrogen atom were a stadium
Blow a hydrogen atom up to 120 meters across and its proton would be a grain about 2 millimeters wide at the center.
Scale Guide
From electron to atoms, to scale
Explore the scaleNuclei occupy femtometers; atoms extend tens of thousands of times farther because their size is set by electron states rather than the nuclear core.
Shared scale
Each step uses a different kind of size
Shared physical scale
385pm
These values do not all describe hard edges. The electron is represented by an experimental upper bound; nucleon sizes come from scattering measurements; nuclear size describes a distribution of nuclear matter; atomic diameter depends on how the electron cloud is defined.
From the electron’s upper bound to the cesium atom, the lineup spans more than nine powers of ten.
First threshold
The electron has no measured size
No internal structure has been detected in the electron, so its plotted value is an upper limit. Protons and neutrons are composite baryons made of quarks and gluons; scattering experiments resolve their femtometer-scale distributions, although a neutron does not have a simple charged outer surface.
Nuclear scale
A heavy nucleus is only about nine times wider than a proton
The plutonium-244 nucleus holds 94 protons and 150 neutrons, 244 times as many particles as a lone proton. Nucleons pack at nearly constant density, so nuclear radius grows only roughly with the cube root of the nucleon count.
Shared physical scale
12.2fm
Atomic threshold
The biggest jump is from nucleus to atom
A hydrogen atom is one proton and one electron. Its characteristic diameter is about 60,000 times the proton’s, and almost all of that volume is the electron’s probability cloud rather than packed material.
Shared physical scale
79.1pm
Atomic spread
Atomic size depends on the convention
Atomic radii depend on the convention and chemical environment. Under the convention used here, neutral cesium extends much farther than hydrogen because its outer electron occupies a higher shell. A microwave transition in cesium-133—not the atom’s size—defines the SI second.
Shared physical scale
442pm
Scale anchors
Worth remembering
The nucleus carries almost all the mass
More than 99.9% of an atom’s mass sits in its nucleus, even though the nucleus fills a tiny fraction of the atom’s volume.
Protons and neutrons are nearly the same size
In the values used here, the proton is 1.68 femtometers across and the neutron 1.72, a difference of about 2%.
More protons do not mean a bigger atom
Cesium has 55 protons to hydrogen’s one, and its atom is about five times wider. Uranium, with 92 protons, is smaller than cesium.
Profiles
The six objects, one by one
Definitions, quoted sizes and caveats for each particle, nucleus and atom above.
The electron is the fundamental particle that gives atoms their chemistry and matter its electric currents. Experiments show no evidence of any internal structure, so its size is represented here by an upper bound rather than a measured diameter.
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The proton is the positively charged particle that gives every atomic nucleus its identity. Less than 1% of a proton’s mass comes from the masses of its quarks; almost all of the rest comes from the energy of quarks and gluons bound by the strong force.
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The neutron is the electrically neutral baryon that, together with the proton, builds atomic nuclei. A free neutron decays by beta-minus decay into a proton, an electron, and an electron antineutrino in about 15 minutes, while neutron-rich matter can persist inside neutron stars.
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The plutonium-244 nucleus is the heavy atomic nucleus of a long-lived plutonium isotope, built from 94 protons and 150 neutrons. It was forged in rare neutron-rich explosions before the Solar System formed, and its 81.3-million-year half-life made it an important extinct radionuclide in early Solar System history.
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Hydrogen is the universe’s simplest and most common atom. Most hydrogen nuclei were forged in the first minutes after the Big Bang, and neutral hydrogen atoms emerged only after the universe cooled enough for electrons and protons to combine about 380,000 years later.
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Cesium is a large alkali-metal atom with a loosely held outer electron. The fixed frequency of a hyperfine transition in cesium-133 defines the SI second, linking atomic physics to every precise measurement of time.
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Chemistry happens at the outer edge of this lineup. Bonds, colors and reactions involve the electrons around atoms, about a tenth of a nanometer out; the nucleus, tens of thousands of times smaller, matters mainly in radioactivity and nuclear reactions.
Continue with another guide or open the full scale.
About
Scale of Space is a scroll-based journey through the universe, placing objects on a single logarithmic scale so you can compare size across an unbroken range.
Guides explain selected parts of that range, while focused views filter the objects by subject.