Salam. Sorry for the late reply for the answers to previous questions.
Ok, the answer to why alloy is harder than its's pure metal is:
1. Atoms of foreign element added to metal are of different size from atoms in pure metal.
2. These atoms will disrupt the orderly arrangement of the atoms in metal.
3. When force is applied, the foreign atoms will reduce the sliding of the atoms over each other.
4. Thus making alloy harder and stronger.
Pure metal is soft, malleable and ductile because:
1. Atoms are of the same size
2. Atoms are arranged in layers and orderly manner.
3. When force is applied the layers of atoms can easily slide over each other
Note: make sure you know how to draw the diagram for the arrangement of atoms in alloy and pure metal