Three borides are known: UBr₂, UBr₃, and UBr₄. The most attention has been paid to the tetraboride, the properties of which are: melting point, >2,100 degrees C (but oxidizes rapidly above 600 degrees C); sp gr# = 9.38 g/mL; thermal expansion, 7.1 X 10_{- 6}(20 to 1,000 degrees C); modulus of rupture (20 degrees C), 60,000 psi (414 MPa); electrical resistivity, 3 X 10_-5ohm.cm.