What factors account for the hardness and high melting points of covalently bonded substances?

Answer 1

There are two types of structures that utilize covalent bonding: molecular covalent structures and network covalent structures. Molecular covalent structures are chemicals that arrange themselves into discrete molecules at the atomic level and interact with each other, through intermolecular forces (dispersion forces, dipoles, H-bonding, etc.), to form solids or liquids, the classic example being the "Mickey Mouse" structure of a water molecule. These chemicals usually display the opposite of what you are describing: they have the lowest melting/boiling points and are not typically "hard surfaced" due to the weakness of the intermolecular forces mentioned above.

You are, I assume, talking about a network covalent bond. These bonds consist of a lattice of covalently bonded atoms, the classic example being a carbon structure where each carbon atom is covalently bonded to four other carbon atoms to form one huge lattice. These covalent bonds are very strong, meaning they are not broken easily by heat (explaining the high melting points) or by force (explaining the hardness/sturdiness). Examples of this structure's bonding at work are in diamond, or in sand (sand, SiO2, is a weaker example of this).