I spotted recently that the commonest isotope of Li is Li-7 (atomic no=3, mass number = 7). This means that the nucleus contains 4 neutrons. According to conventional wisdom the 4th neutron is not necessary, because the nuclear stability only requires 3 neutrons. This pattern certainly holds true of all other light elements where the no. of protons = the no. of neutrons in their most abundant isotope. Can anyone explain the anomalous nature of lithium?
Lithium is not anomalous in behaviour. Li-7 being more common than Li-6 is explanable by Big-Bang Nucleosynthesis (there are two pathways I know of for Generating Li-7). Lithium 6 on the other hand is very difficult to make in the big-bang, it requires the fusion of He-4 and Deuterium which has a very small cross section (it's not very likely to happen except in a very small proportion of collisions as detailed inPrimordial synthesis of Li-6 and the rate of alpha-/d, gamma/ Li-6. Lithium 6 is as stable as Lithium 7, it's just that as it turns out it takes very little energy to turn Lithium 6 into Tritium. So why is there more Lithium 7 than Lithium 6 in the universe? It's not because Lithium 7 is stable and Lithium 6 isn't, it's just because Lithium 7 is easier to make in a big bang than lithium 6 is.
