Isotopes of lithium

Lithium is the lightest solid element and has two main stable forms, called isotopes: lithium-6 (⁶Li) and lithium-7 (⁷Li). On Earth, about 7.6% of natural lithium is ⁶Li and about 92.4% is ⁷Li. Because of this mix, lithium’s average atomic weight is around 6.94, though it can change slightly depending on where it is found. In addition to these stable isotopes, lithium also has several radioactive isotopes, but they break down quickly and are not common in nature.^[1][2]

Lithium-6 is important because it can easily absorb neutrons. When it captures a neutron, it splits into tritium (³H) and helium-4 (⁴He). This reaction is very valuable in nuclear technology.^[3] It is used in nuclear weapons to make fission bombs stronger and in fusion research to produce tritium, which is one of the fuels needed for experimental fusion reactors.^[4] Lithium-7, even though it is more common, is also very useful. In nuclear power plants, lithium hydroxide made with extra ⁷Li is added to the water in reactors. This helps control the water’s pH. Using mostly ⁷Li is important because if too much ⁶Li were present, it would absorb neutrons and make unwanted tritium, which could cause problems in the system.^[1][5]

Lithium isotopes are also helpful in Earth science. The ratio of ⁶Li to ⁷Li can change when rocks form, when hot water interacts with minerals, or even when living things absorb lithium. Scientists can study these changes to learn about weathering, plate tectonics, and even ancient climate conditions.^[6] In space science, lithium has another mystery. Theories about the Big Bang say that there should be more ⁷Li in old stars than we actually see. This difference is called the “cosmological lithium problem,” and scientists are still trying to solve it.^[7]

Some radioactive forms of lithium, like lithium-8 (⁸Li), decay very quickly. Lithium-8 lasts less than a second before breaking apart into other particles. While not useful in everyday technology, these unstable isotopes are studied in physics experiments and sometimes tested for medical research. Separating lithium-6 from lithium-7 is very difficult because their masses are close, only about a 16.7% difference. During the Cold War, a method called COLEX, which used lithium mixed with mercury, was developed for isotope separation. Today, scientists are working on new techniques, including special chemical methods, lasers, and advanced membranes.^[1]

Because of its nuclear uses, lithium-6 is considered a strategic material, especially for future fusion reactors like ITER that will need tritium fuel. Meanwhile, lithium-7 continues to be widely used in nuclear reactor systems, heat-transfer salts, ceramics, and, most importantly, in rechargeable lithium-ion batteries. For most battery uses, the exact isotope mix does not matter, but in nuclear and geological studies, it can make a big difference.^[1]