Simple Molecular Substances.
SIMPLE MOLECULAR SUBSTANCES
Simple molecular substances are gases, liquids or solids with low melting and boiling points.
In simple molecular substances, the atoms form very strong covalent bonds to form small molecules of two or more atoms. However the attraction between these molecules is very weak. It is because of this weak attraction that the melting and boiling points are so low.
- Molecular Substances do not conduct electricity, because there are no ions.
For example, water atoms (H2O) have strong covalent bonds within them (between the Oxygen and Hydrogen.) However, the bonds between it and other water molecules are weak. You can usually tell whether a substance is a simple molecular one due to its physical state, which is always 'mushy.'
The Halogens are all simple molecular. Their melting and boiling points increase as you go down the group. This is because the molecules get bigger and therefore the weak bonds get gradually stronger.
Giant Covalent Structures.
GIANT COVALENT STRUCTURES
Giant Covalent Structures are structures with atoms that are all bonded to each other by strong covalent bonds.
Also, they all have high melting and boiling points. These are some examples of giant covalent structures:
Diamond is the hardest natural substance on earth. Each carbon atom forms 4 covalent bonds in a very rigid covalent structure, this is why it's so hard. There are many strong covalent bonds, so its got very high melting and boiling points.
Graphite is a very good conductor of electricity. Each carbon atom only forms 3 covalent bonds, which creates sheets of carbon atoms which are free to slide over each other. This means the layers of graphite can be rubbed off onto paper- thats how pencils work. Graphite also has high melting and boiling points, as the covalent bonds need loads of energy to break.