Lithium-6 and lithium-7 are both stable isotopes of lithium. Each has three protons; lithium-6 has three neutrons, while lithium-7 has four. That extra neutron changes the atom’s mass and its usefulness in certain nuclear applications, but it does not make lithium-7 a different element. Natural lithium is mostly lithium-7, while lithium-6 is especially useful for capturing thermal neutrons.
How lithium-6 and lithium-7 differ
An isotope’s number identifies the total number of protons and neutrons in its nucleus. Because both isotopes have three protons, both are lithium. Their mass numbers differ because lithium-7 has one more neutron.
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| Property | Lithium-6 | Lithium-7 |
|---|---|---|
| Protons | 3 | 3 |
| Neutrons | 3 | 4 |
| Relative atomic mass | 6.0151228874(16) | 7.0160034366(45) |
| Natural isotopic composition | 0.0759(4), about 7.59% | 0.9241(4), about 92.41% |
| Stability | Stable | Stable |
| Notable distinction in the applications covered here | Strong thermal-neutron capture; used in neutron shielding and measurement, and in fusion tritium-breeding concepts | Dominant isotope in natural lithium; enriched lithium-7 is listed as a specialized isotope product |
The relative atomic masses and natural compositions are from the National Institute of Standards and Technology’s lithium isotope reference. Parenthetical digits are part of NIST’s reported uncertainty notation. Natural composition describes the isotope mix in ordinary lithium; it is not the composition of an enriched product.
Why lithium-6 is useful for neutron applications
The key application difference is nuclear: lithium-6 has a large thermal-neutron capture cross section. NIST reports an approximate value of 941 barns in its 2018 publication on lithium-6-enriched neutron-shielding glass. The main reaction is ⁶Li(n, α)³H, which produces an alpha particle and tritium. NIST notes a small prompt-gamma branch as well. This makes lithium-6 useful in particular neutron-related settings; it does not mean lithium-7 has no neutron reactions.
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Shielding
NIST describes lithium-6-enriched silicate glass as a common slow-neutron shielding material used at several neutron research facilities. Its neutron-capture reaction is the basis for its role in this specialist application. The approximate cross section above is the figure reported in the NIST publication, not a matched comparison with lithium-7.
Neutron depth profiling
Neutron depth profiling is a nondestructive measurement technique that uses neutron reactions, including reactions with lithium-6, to determine the amount and distribution of an element in a material. NIST describes its use in lithium-ion battery research to profile lithium within a cell. This is a measurement application; it does not show that consumer batteries are enriched in lithium-6.
What the isotopes are used for
Lithium-6 and fusion tritium breeding
Deuterium-tritium fusion concepts need tritium, and the U.S. Department of Energy says tritium-breeding systems will require enriched lithium-6. The isotope can produce tritium when it captures a neutron. DOE identifies scalable lithium-isotope separation as a research challenge because lithium-6 is much less abundant naturally. This describes a fuel-cycle requirement under development, not routine commercial fusion-power generation.
Lithium-7 and specialized supply
Lithium-7 makes up about 92.41% of natural lithium according to NIST. The DOE National Isotope Development Center lists stable lithium-7 with enrichment above 99.5 atom percent. Those facts establish its abundance and a listed specialized supply specification; they are not a complete catalogue of lithium-7 applications or a guarantee of retail availability.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Natural abundance versus enrichment
Natural abundance is the proportion of each isotope in ordinary lithium. Enrichment is a process that raises the proportion of a chosen isotope in a product. The two percentages answer different questions and should not be compared as if they described the same material.
| Measure | Lithium-6 | Lithium-7 |
|---|---|---|
| Natural proportion, NIST | About 7.59% | About 92.41% |
| Enrichment listed by DOE’s National Isotope Development Center | 95–99 atom percent | Above 99.5 atom percent |
The enrichment ranges are catalog specifications from the DOE National Isotope Development Center’s lithium listing. They do not establish universal supply, delivery availability, or price.
Which isotope matters in a given context?
- For ordinary natural lithium: lithium-7 is the more abundant isotope.
- For documented thermal-neutron capture uses: lithium-6 is the relevant isotope in the shielding, measurement, and tritium-breeding contexts described above.
- For lithium-ion battery research: neutron depth profiling can measure lithium distribution; that does not imply a battery uses enriched lithium-6.
- For isotope-specific material: enrichment specifications are specialist catalog information, not a consumer-product comparison.
For further detail, NIST explains neutron depth profiling and its lithium applications; DOE outlines lithium-6’s role in deuterium-tritium fusion fuel; and NIST’s 2018 paper covers lithium-6-enriched neutron-shielding glass.
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