解答此题：---**Question Stem:** 12. 磷青铜是铜与锡、磷的合金,质地坚硬,主要用作耐磨零件和弹性元件。某立方磷青铜晶胞结构如图所示,晶胞参数为 a pm($N_A$ 表示阿伏伽德罗常数的值)。下列说法正确的是 **Options:** A. $Cu^+$ 的价电子排布式为 $3d^9 4s^1$ B. Cu 与 Ni 在元素周期表中都位于 ds 区,属于过渡元素 C. 该晶体中,距离 Sn 原子最近的 Cu 原子有 8 个 D. 该晶体密度计算式为 $\frac{342}{N_A \times (a \times 10^{-10})^3}$ g·cm$^{-3}$ **Chart/Diagram Description:** * **Type:** Crystal structure diagram (unit cell). * **Main Elements:** * A cube represents the unit cell. * There are three types of atoms represented by circles: * White circles labeled "Sn" are located at the 8 corners of the cube. * Black circles labeled "Cu" are located at the center of each of the 6 faces of the cube and also at the center of the cube (body-centered). * A dashed circle labeled "P" appears to be close to one of the white corner atoms, potentially indicating its position relative to the corner or another atom, though its exact position within the unit cell structure shown is unclear from the diagram itself. However, the structure seems to depict a body-centered cubic (BCC) type arrangement of black circles within a framework defined by the white circles at the corners and black circles on the faces. The dashed lines suggest connections or relationships within the inner structure. * Lines connect the atoms, forming a network within the cube. Solid lines connect the corner atoms to adjacent face-centered atoms and the body-centered atom. Dashed lines also connect some atoms. * Labels Sn, Cu, and P are placed next to representative atoms. * The diagram shows a cubic lattice framework with specific positions occupied by different atoms. The white circles (Sn) are at the vertices. The black circles (Cu) are at the face centers and the body center. The P atom is indicated by a dashed circle near one vertex.