Types of substances

Hard, brittle, high melting point due to strong electrostatic attraction , conduct electricity only when molten or in solution since as a liquid, electrons are free to carry charge

Need covalent bonding, more than one atom, smaller, low melting and boiling points because of weak intermolecular forces, can't conduct electricity due to no free electrons.

Graphite
Diamond

network of carbon atoms that form 3 covalent bonds for each carbon atom

network of carbon atoms that form 4 covalent bonds for each carbon atom

Lustrous (shiny)
Hard.
High density (are heavy for their size)
High tensile strength (resist being stretched)
High melting and boiling points.
Good conductors of heat and electricity.

giant structure
very strong forces of electrostatic attraction between positive metal ions and negative electrons

Due to its 3 covalent bond, it has a layered structure and not all electrons are held in place and can carry a charge.
Also cheap and unreactive

There are weak forces between the layers which allow the layers to slide past each other

Diamond is very hard because all the carbon atoms are joined by 4 strong covalent bonds.

3 bonds for each carbon, an example is C60
low boiling point, soft and slippery
similar to graphene (fixed carbon atoms with no fixed formula, very light but strong and conducts)