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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Lithium-6 and lithium-7 are both stable isotopes of lithium. Each has three protons, but lithium-6 has three neutrons while lithium-7 has four. That extra neutron changes the isotope’s mass and its behavior when it encounters neutrons; it does not make lithium-7 a different element. Natural lithium is mostly lithium-7, while lithium-6 is especially useful in certain neutron-shielding, measurement, and fusion-fuel-breeding contexts.
How lithium-6 and lithium-7 differ
An isotope’s number is its total number of protons and neutrons. Both isotopes have three protons, which makes them lithium; their neutron counts account for the difference between the numbers 6 and 7.
| 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% |
| Stable? | Yes | Yes |
The mass and natural-composition figures are from the National Institute of Standards and Technology (NIST) current reference, accessed in 2026; the parenthetical digits are part of NIST’s uncertainty notation. NIST: Atomic Weights and Isotopic Compositions for Lithium.
Why the neutron difference matters
The additional neutron gives lithium-7 a greater atomic mass. More importantly for the applications covered here, 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 reported principal reaction is ⁶Li(n, α)³H, which produces an alpha particle and tritium; the publication also notes a small prompt-gamma branch. NIST: Investigation of Alteration of Li-6 Enriched Neutron Shielding Glass.
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This is a specific advantage for thermal-neutron capture applications, not evidence that lithium-7 has no neutron reactions. A matched lithium-7 capture value is not established by the NIST passage cited here, so the comparison should not be treated as a complete set of neutron data.
What lithium-6 is used for
Neutron shielding
NIST describes lithium-6-enriched silicate glass as a common slow-neutron shielding material at several neutron research facilities. Its usefulness comes from capturing neutrons through the reaction described above.
Neutron depth profiling
Neutron depth profiling is a nondestructive measurement method that uses neutron reactions, including a reaction with lithium-6, to measure the amount and distribution of lithium in materials. NIST describes its use in lithium-ion battery research to profile lithium within a cell. This is a research and measurement application; it does not mean consumer batteries are enriched in lithium-6. NIST: Detecting the Flavors of Important Elements With Neutron Depth Profiling.
Tritium breeding for fusion fuel systems
In deuterium-tritium fusion concepts, tritium-breeding systems require enriched lithium, specifically lithium-6. The U.S. Department of Energy 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. DOE Office of Science: DOE Explains…Deuterium-Tritium Fusion Fuel.
What lithium-7 is known for
Lithium-7 is the dominant isotope in natural lithium, making up about 92.41% according to NIST’s current composition reference. DOE’s National Isotope Development Center lists lithium-7 as a stable isotope product enriched to above 99.5 atom percent. These sources establish its abundance and the existence of a specialized enriched product, but they do not provide a basis for an exhaustive list of lithium-7 applications. DOE National Isotope Development Center: Lithium.
Natural abundance is not enrichment
Natural composition describes the isotope mix in ordinary lithium. Enrichment describes a processed product in which the proportion of a selected isotope has been raised. The figures below compare NIST’s natural-composition values with the enrichment specifications listed by DOE’s isotope catalog; they are different kinds of measurements.
| Isotope | Natural composition (NIST) | Listed enriched product (DOE catalog) |
|---|---|---|
| Lithium-6 | About 7.59% | 95–99 atom percent |
| Lithium-7 | About 92.41% | Above 99.5 atom percent |
The catalog specifications do not establish universal supply, retail availability, price, or delivery terms.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which isotope matters for a particular purpose?
- For ordinary lithium composition: lithium-7 is the majority isotope in nature.
- For the documented thermal-neutron capture applications: lithium-6 is the relevant isotope, including certain shielding and measurement uses.
- For deuterium-tritium fusion fuel breeding concepts: DOE identifies enriched lithium-6 as a requirement.
- For a consumer choosing a lithium-ion battery: this isotope comparison does not identify a meaningful consumer alternative; the cited lithium-6 battery work concerns a research measurement technique.
These are scientific and application-specific distinctions, not a general ranking of one isotope as better than the other.
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