Why is pure copper relatively soft?

Pure copper is considered a relatively soft and ductile metal because it is not inherently strong or hard in its natural state. Its mechanical properties are defined by its crystal structure, which is face-centered cubic.12

In metallurgy, the strength of a metal is determined by its resistance to the slipping of one plane of atoms over another. Pure copper lacks the internal barriers—such as alloying elements, fine precipitates, or the high density of dislocation tangles created by cold-working—that are necessary to inhibit this atomic slip.34

While pure copper is soft, its strength and hardness can be significantly increased through cold-working, which introduces structural distortions that act as barriers to slip. Conversely, these hardened states can be reversed through annealing, a heat-treating process that restores the metal's original softness.142