Introduction
In metals, resistivity is a measure of how strongly a material opposes the flow of electric current. It is composed of two parts - one part that is characteristic of the particular substance and another part that is due to crystal imperfections.

Explanation
Characteristic resistivity
The characteristic resistivity of a particular substance is determined by its atomic and electronic structure. The arrangement of atoms in a metal determines its electrical conductivity. Metals have a high electron density and a delocalized electron structure, which allows for the easy flow of electric current. This is why metals generally have low resistivity.

Crystal imperfections
Crystal imperfections, such as dislocations, grain boundaries, and impurities, can significantly affect the resistivity of a metal. These imperfections create obstacles in the path of electrons, hindering their movement and increasing the resistivity of the material.

- Dislocations: Dislocations are defects in the crystal lattice structure of a metal. They occur due to the presence of extra or missing atoms in the lattice. Dislocations create localized regions of strain, which disrupt the regular flow of electrons and increase resistivity.

- Grain boundaries: Grain boundaries are interfaces between different crystal grains in a metal. These boundaries can impede the flow of electrons, leading to increased resistivity. The presence of grain boundaries is more common in polycrystalline metals.

- Impurities: Impurities are foreign atoms or molecules that are unintentionally present in a metal. These impurities can disrupt the regular arrangement of atoms and create localized regions of strain. This strain affects the flow of electrons and increases resistivity.

Conclusion
In summary, resistivity in metals is composed of two parts - one part that is characteristic of the substance and another part that is due to crystal imperfections. The characteristic resistivity is determined by the atomic and electronic structure of the metal, while crystal imperfections such as dislocations, grain boundaries, and impurities can increase resistivity by hindering the flow of electrons.