Question

Which of the following has hcp structure?
A. Al
B. Mg
C. Cu
D. Ni

Answer 1

Hint: The particles in any atom are arranged in a certain packing or arrangement in their respective crystal lattices. HCP structure stands for hexagonal close packed structure. These structures have a crystal with particles arranged in a shape of hexagon.

Complete step-by-step answer: A unit cell is the smallest part of the crystal lattice. Unit cells are present in various places in a structure. Like for Face centered unit cell FCC, the unit cell is present on all faces which is shared by each face. The unit cells together make up the arrangement of the lattice. Particle layers are arranged in various patterns which contribute to the making of crystal lattice. This is in the form of packing. In hexagonal packing the spheres in the first layer are arranged adjacent to each other while those of the second layer are arranged in the depression of the first layer.
Three- dimensional close packing, where the hexagonal packing is seen in the first two layers of the arrangement, while a third layer is placed in such a way that it covers, tetrahedral voids as well as octahedral voids.
These voids arise when the spheres are placed in the first and second layer. The packing that covers all tetrahedral voids is known as hexagonal close packing in 3-D (HCP), while when the third layer covers all the octahedral voids, it is called cubic close packing in 3-D (CCP).
The packing in HCP has a unit cell of Z = 6, while in CCP the unit cell is Z = 4.
The metals that are a part of HCP crystal are Zinc, Magnesium, and Cadmium. While metals that are a part of CCP are, Copper, Silver, and Gold, which are coinage metals.

Hence, magnesium, Mg is the metal that has HCP structure, so option B is correct.

Note: The packing tells us the layers arranged in a crystal lattice. For hexagonal it is arranged in a way that the first layer is simple spheres, then the second layer is arranged in depression of the first layer, this creates voids that are covered by the third layer, hence these give rise to HCP and CCP structures.