why deos magnesium show hcp but not copper

 

• The crystal structure adopted by a particular metal at a given temperature and pressure will be the structure which provides for the minimisation of free energy.
• The atoms of the metal will form arrangements which ensure that the electron orbitals that bond them have the repulsion/attraction due to bond length all contribute to the lowest energy state. 
• Each metal has a slightly different metallic radius, which is defined as half of the distance between the two adjacent metal ions in the metallic lattice.
• Depending on the metallic radius, the lowest energy state would be achieved by different atomic arrangements. 
• For the alkaline earth metals with the smaller metallic radii (beryllium and magnesium), the lowest energy state is the HCP structure.
• To achive lowest energy level, copper forms FCC.