Uranium-235 vs Uranium-238
U-235 and U-238 are both uranium because both have 92 protons. Their different neutron counts change their masses, decay rates and response to neutron-induced fission.
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Uranium-235 vs Uranium-238 in one minute
U-235 and U-238 are isotopes of the same element. Both nuclei contain 92 protons; U-235 has 143 neutrons and U-238 has 146. In normal terrestrial uranium, U-238 is overwhelmingly more abundant. Both are radioactive, but U-235 is fissile with low-energy neutrons whereas U-238 is classed as fissionable rather than fissile in that sense.
Same atomic number means same element. Changing neutron number can still change nuclear stability and nuclear-reaction behavior dramatically. “Radioactive,” “fissile,” and “fissionable” are not interchangeable words.
See how the idea connects
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Both nuclei are uranium because atomic number stays 92.
Different neutron counts give mass numbers 235 and 238.
U-238 dominates normal terrestrial uranium; U-235 is a much smaller fraction.
Their responses to neutron capture and fission are not identical.
Why are both isotopes still uranium?
Element identity is fixed by proton number. Uranium has atomic number 92, so every uranium isotope has 92 protons.
| Nuclide | Protons | Neutrons | Mass number |
|---|---|---|---|
| U-235 | 92 | 143 | 235 |
| U-238 | 92 | 146 | 238 |
The neutron difference changes nuclear properties without changing the chemical element.
Which isotope is more common in natural uranium?
The Commission on Isotopic Abundances and Atomic Weights lists representative amount fractions for normal terrestrial uranium of about 0.7204% U-235 and 99.2742% U-238, with a very small U-234 component. Natural variation exists, so these are reference composition values rather than a claim that every sample is identical.
This is why a sample of ordinary natural uranium is dominated by U-238 even though U-235 receives disproportionate attention in discussions of fission.
Are both U-235 and U-238 radioactive?
Yes. Both are primordial radioactive nuclides and both undergo alpha decay, but their half-lives differ. That means equal numbers of U-235 and U-238 nuclei would not have the same activity.
Radioactive decay is spontaneous. It should not be confused with neutron-induced fission, which is a different nuclear process.
For the broader concepts, see the Radioactivity lesson and Isotopes lesson.
What is the conceptual difference between fissile and fissionable here?
The U.S. Nuclear Regulatory Commission distinguishes fissile from the broader category fissionable. U-235 can undergo fission after capture of low-energy thermal neutrons and is therefore fissile. U-238 can undergo fission with sufficiently high-energy neutrons but is not fissile in the thermal-neutron sense.
This page keeps the distinction conceptual. It does not provide enrichment procedures, reactor operating parameters or instructions for handling nuclear material.
Do U-235 and U-238 have different ordinary chemistry?
The isotopes have the same proton number and, for neutral atoms, the same number of electrons. Their ordinary electron configurations and chemical identities are therefore extremely similar. Isotope mass can cause small isotope effects, but the dramatic contrast discussed on this page is primarily nuclear rather than chemical.
Return to the Uranium guide for crystal phases, occurrence, history, safety framing and source-backed element data.
Why is uranium used in nuclear power?
Uranium is useful because nuclear fission can release energy from binding-energy differences in heavy nuclei. Uranium-235 is fissile with thermal neutrons, so a captured neutron can lead to fission and additional neutrons. Natural uranium is mostly U-238; U-238 is not the same thermal-neutron fuel, but it is fertile and can contribute to reactor fuel cycles after neutron capture and subsequent nuclear transformations.
This is a nuclear-property explanation. It should not be confused with uranium’s ordinary chemical reactivity, and it does not require operational reactor or enrichment instructions.
What students often mix up
U-235 and U-238 are not different elements; both have atomic number 92.
Radioactive is not a synonym for fissile. Both isotopes are radioactive, but their neutron-induced fission behavior differs.
Natural abundance percentages are reference composition values for normal terrestrial material and can show small natural variation.
This comparison is educational, not operational guidance for enrichment, reactor design or nuclear-material processing.
U-235 and U-238 have almost the same electron chemistry but very different neutron-induced nuclear behavior.
Check your understanding
Answer before opening the explanation. The aim is understanding, not speed.
1How many neutrons are in U-235?
143, because 235 − 92 = 143.
2Why is U-238 still uranium even though its mass number differs from U-235?
Both have 92 protons, and proton number defines the element.
3Does “both are radioactive” mean their fission behavior is identical?
No. Radioactive decay and neutron-induced fission are different nuclear properties, and U-235 and U-238 behave differently in fission.
4What makes U-235 especially important in many reactors?
It can undergo fission after absorbing a thermal neutron, releasing energy and additional neutrons.
Sources and terminology
Definitions and reference claims are anchored to authoritative scientific organizations and peer-reviewed literature where needed. Element Lookup adds teaching explanation, examples and visual structure; it does not treat AI as the source of scientific definitions or numbers.
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