Why the theoretical density is greater than the actual density of nickel?

Answer 1

When you consider the theoretical density of a material, you consider a material where every atomic position is filled with an atom of that material in a perfect lattice arrangement with no errors. For example, if you consider Nickel, each of the atomic positions are filled with a Nickel atom. So if this were to occur over 1g/cm^3, you have a theoretically dense Nickel. However, in this universe this is virtually impossible, as we know it anyway! :)

Real world materials, no matter how brilliant your processing parameters always contain some impurities (other atoms besides Nickel), flaws, missing atoms (called a vacancy), porosity, etc... All of these things bring the density of the Nickel away form the theoretical density and you end up with the "real world" density of the material. This is a basic explanation, but I hope it gives some information and helps in kicking off some research on the net.

Materials Scientist, PhD.

Answer 2

Theoretical density is calculated based on the assumption that all atoms are perfectly arranged in a crystal lattice, leading to a higher value. In reality, imperfections like vacancies and dislocations in the crystal lattice cause the actual density to be lower than the theoretical density. These imperfections increase the space between atoms, reducing the overall density of the material.