Pure iron, also known as wrought iron, has a body-centered cubic (BCC) crystal structure, which makes it relatively soft and not very strong. The weakness of pure iron is due to the lack of bonding between the iron atoms and the low density of the crystal structure.

When carbon is added to iron, it forms iron carbide (Fe3C), which has a crystalline structure known as cementite. Cementite has a higher density and stronger bonds between the atoms than the BCC structure of pure iron. This increases the strength and hardness of the iron.

Additionally, the presence of carbon also affects the way in which the iron atoms are arranged in the crystal structure. In steel, the carbon atoms are distributed in the form of tiny precipitates (particles) throughout the iron matrix. These precipitates hinder the movement of the iron atoms and prevent the growth of slip planes and dislocations, which are the main causes of plastic deformation and failure.

In summary, the addition of carbon to iron changes its properties by creating a new crystal structure with stronger bonds and a higher density, and by introducing precipitates that prevent the movement of the iron atoms and enhance the strength of the material.