Why are alloys generally harder than pure metals?

Alloys are generally harder than pure metals primarily due to the disruption of the regular atomic arrangement in the metal layers caused by the presence of different sized atoms. When different elements are combined to form an alloy, the atoms of the added element occupy positions within the metallic lattice of the base metal. This creates irregularities in the structure, as the various atomic sizes prevent the layers of atoms from sliding over each other easily.

In pure metals, the atoms are typically uniform in size and can move past each other relatively easily, which contributes to their malleability. However, when alloys are formed, the mixed atomic sizes resist this sliding motion, making it more difficult for the layers to shift, and resulting in increased hardness. This is why the disruption of metal layers due to differing atomic sizes in alloys contributes to their greater hardness compared to pure metals.

The other choices, while relating to properties of metals or alloys, do not specifically explain the increased hardness mechanism. For instance, a higher melting point does not directly affect hardness, and delocalized electrons are more related to conductivity rather than hardness. Malleability is actually decreased in many alloys compared to pure metals, further clarifying why the correct answer focuses on the disruption of the atomic layers.