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Ionic crystalline substances are made up of a highly ordered closely packed lattice network of positive and negative ions held together by electrostatic force of attraction. Solvent molecules (solvent of crystallization) are sometimes accommodated inside these closely knit networks through their own polar ends. CuSO4, 5H2O is an example. When this ordered network is broken down by some external intervention like mechanical stress, the positive and negative ions are displaced from their positions, hence lattice energy is compromised and the crystal structure is lost. Just another random arrangement of ions will not be able to bring about the same stability. This is the reason for non-malleability and brittleness of these crystalline substances. Metallic crystals, on the other hand, are made up of a non-ending kernel of positively charged core ions surrounded by a continuous ‘sea’ of electrons. When some ions of the core are replaced or rearranged by some other set of ions from the same metal, through its length or breadth (or even of a different metal, as in the case of alloys), the sea of electrons still surround them equally well and hence stability is not compromised at all. This is the reason for malleability and ductility of metals.
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