<P> where ρ is density, N Avogadro constant, and A the atomic mass . </P> <P> It is the simplest point defect . In this system, an atom is missing from its regular atomic site . Vacancies are formed during solidification due to vibration of atoms, local rearrangement of atoms, plastic deformation and ionic bombardments . </P> <P> The creation of a vacancy can be simply modeled by considering the energy required to break the bonds between an atom inside the crystal and its nearest neighbor atoms . Once that atom is removed from the lattice site, it is put back on the surface of the crystal and some energy is retrieved because new bonds are established with other atoms on the surface . However, there is a net input of energy because there are fewer bonds between surface atoms than between atoms in the interior of the crystal . </P>

List four conditions that can cause vacancies to form during solidification of a metal