What determines the crystal structure taken by metals? For example Sr has FCC and Ba BCC. The packing fraction is greater for FCC. However, due apparently to coordination number and number of "next nearest" neighbors, BCC is favored in the case of Barium despite its lower packing fraction. The introductory texts I am reading claim metal bonds are mostly isotropic. Sr and Ba have the same valence shells. Nevertheless, is electron structure the key to determining bond geometry?