Final answer:
The structure of metals, consisting of a regular arrangement of atoms and a sea of delocalized electrons, is responsible for their characteristic hardness and high melting points due to the strong metallic bonding.
Step-by-step explanation:
The hardness and high melting points of metals can be attributed to their unique metallic bonding and crystalline structure. In metals, atoms are arranged in a regular pattern and their outer electrons are delocalized, creating a 'sea' of electrons. This structure allows the atoms to slide over one another without breaking the metallic bonds, giving metals their malleability. However, the electrostatic attraction between the positively charged atomic nuclei and the sea of delocalized electrons is strong, contributing to the hardness and high melting points of metallic solids.
Since the melting point is dependent on the strength of these metallic bonds, we see a wide range of melting points across different metals. For example, mercury (Hg) is liquid at room temperature, whereas tungsten (W) has one of the highest melting points. The thermal and electrical conductivity, along with metallic luster and malleability, are also key characteristics that result from the metallic bonding and closely packed atomic arrangement within metals.