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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesLithium-6 and lithium-7 are two stable forms of lithium. Each has three protons, but lithium-6 has three neutrons and lithium-7 has four. That extra neutron gives lithium-7 a greater mass; it does not make it a different element.
What makes lithium-6 and lithium-7 isotopes?
An isotope is an atom of an element with the same number of protons as other atoms of that element but a different number of neutrons. Lithium’s atomic number is 3, so both isotopes have three protons. The number after the hyphen is the mass number: the total number of protons and neutrons.
| Property | Lithium-6 | Lithium-7 |
|---|---|---|
| Protons | 3 | 3 |
| Neutrons | 3 | 4 |
| Relative atomic mass | 6.0151228874(16) | 7.0160034366(45) |
| Stability | Stable | Stable |
| Representative share of lithium | 7.59(4)% | 92.41(4)% |
The atomic masses and representative isotopic compositions are from the National Institute of Standards and Technology (NIST) Atomic Weights and Isotopic Compositions table, accessed in 2026. Parenthetical digits indicate uncertainty in the final reported digits. The mass numbers 6 and 7 are whole numbers; they are not the precise atomic masses shown in the table.
How common is each isotope?
NIST’s representative composition is about 7.59% lithium-6 and 92.41% lithium-7. These are useful reference proportions, not a guarantee that every sample has exactly the same ratio. The representative values describe materials commonly encountered in laboratories, and lithium isotope ratios can vary between materials. IUPAC documents this variation in its report on isotopic composition of the elements.
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Enriched lithium is a separate case: it contains a deliberately higher proportion of a selected isotope than typical reference lithium. The U.S. Department of Energy’s National Isotope Development Center lithium listing gives catalog enrichment levels of 95–99 atom % for lithium-6 and greater than 99.5 atom % for lithium-7. These are product specifications, not natural abundances, and catalog availability can change.
Why does the isotope difference matter?
Because both isotopes are lithium, they have nearly the same chemistry. Their different masses and nuclear properties can nevertheless matter in measurement and in applications where isotope ratios or nuclear behavior are important. Small differences allow isotopes to be fractionated—separated in relative proportion—by physical, chemical, and biological processes.
Environmental tracing
Researchers can use lithium isotope ratios to investigate the sources of dissolved lithium and environmental processes. IUPAC notes that lithium isotope ratios in water can help distinguish some sources, including water associated with marine sedimentary rocks and water associated with hydrothermally altered igneous rocks. The ratio is one line of evidence, rather than a universal identifier for every water sample.
Selected nuclear applications
The isotopes have distinct roles in some nuclear contexts. IUPAC describes lithium-7 hydroxide monohydrate as a means of helping control coolant pH in pressurized-water reactors. Lithium-6 can produce tritium following neutron capture. These examples reflect their different nuclear properties; they do not change the basic chemical identity shared by both isotopes.
Precision measurement
Isotope differences can also be measured in spectra. NIST’s account of frequency-comb spectroscopy of lithium, published October 3, 2011, describes research measuring differences in spectral emissions between lithium-6 and lithium-7. The work addressed a historical measurement challenge: earlier measurements disagreed substantially, making the small isotope shift difficult to establish.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.The key distinction
Lithium-6 has three neutrons, while lithium-7 has four. Both are stable lithium isotopes with three protons; lithium-7 is heavier and more abundant in NIST’s representative composition, though actual sample ratios can vary.
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