Bulk modulus 42 GPa Mohs hardness 3.0 Brinell hardness 294 MPa CAS registry number 7440-36-0 Most stable isotopes Main article: Isotopes of antimony iso NA half-life DM DE ( MeV ) DP 121 Sb 57.36% 121 Sb is stable with 70 neutrons 123 Sb 42.64% 123 Sb is stable with 72