Why are some metals hard and brittle? What feature of the structure of these metals determines the presence of these properties?

Why are some metals hard and brittle? What feature of the structure of these metals determines the presence of these properties? Which metal is the hardest? Which metals are the softest?

All metals are hard. But the fragility of metals depends on the number of free electrons. A large number of free electrons provides the strength of the layers of individual ions, preventing them from sliding, reducing plasticity.
The hardest metal is chromium, the softest metals are alkali metals.

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