Shear modulus 4.2 GPa Bulk modulus 11 GPa Mohs hardness 0.6 CAS registry number 7439-93-2 Most stable isotopes Main article: Isotopes of lithium iso NA half-life DM DE ( MeV ) DP 6 Li 7.5% 6 Li is stable with 3 neutrons 7 Li 92.5% 7 Li is stable with 4