Covalent substances: Two kinds
Simple molecular substances:
The atoms form very strong covalent bonds to form small molecules of several atoms. But the bonds between the molecules are very weak. Because of this, the melting and boiling points are very low.
Most molecular substances are gases or liquids at room temperature, but they can be solids. Molecular substances don't conduct electricity, as there are no ions. So there is no electrical charge.
Covalent substances: Two kinds 2
Giant Covalent Structures (Macromolecules):
These are similar to giant ionic structures (lattices) except that there are no charged ions. All atoms are bonded together by Strong covalent bonds. This means they have very high melting and boiling points. They do not conduct electricity, even when molten (Except for graphite)
The main examples are Diamond and Graphite, which are both made from Carbon atoms, and Silicon Dioxide.
Covalent substances: Two kinds 3
Each carbon atom contains four covalent bonds in a very rigid giant covalent structure. This structure makes diamond the hardest natural substance, so its used for drill tips
This is what sand is made of. Each grain of sand is one giant structure of silicon and oxygen
Covalent substances: Two kinds 4
Each carbon atom only forms three covalent bonds. This creates layers which are free to slide over each other, like a pack of cards. So they can be rubbed off on to paper (How a pencil works). This is because there are weak intermolecular forces between the layers.
Graphite is the only non-metal which is a good conducter of heat and electricity. Each carbon atom has a delocalised electron and these are what conduct the electricity and heat.