C2aL4 Covalent Bonding
Key Words
Covalent Bond - bond where electrons are shared.
Electrons - tiny particles of negative charge.
Giant covalent structure - lots of atoms bonding to each other with covalent bonds.
Sharing - having equal parts.
Simple molecules - compounds consisting of a few atoms bonded covalently.
Grade E
In reactions with other non-metals, non-metal atoms get full outer shells by a different way. They share electrons to form covalent bonds. This happens in the simplest of molecules or the largest of giant covalent structures.
Hydrogen has one electron. To gain a noble gas structure, we need to have a second electron. Hydrogen atoms therefore share each of their outer electrons.
The fact that one electron is blue and one is purple is there to show the idea. Both electrons from each atom are identical. In some diagrams, you will see one of the electrons represented with a cross.
It is easier to show the hydrogen molecule like this:
The covalent bonding explains why you get H2, not H3.
Grade C
Chlorine behaves in a similar way. It has 7 electrons in the outer shell, so to get a noble gas structure it shares one of its electrons with another chlorine atom.
Notice that we have not shown the inner shells. We can write the molecule like this:
There is a single bond between the chlorine atoms.
Oxygen has 6 outer shell electrons, so two electrons from each atom need to be shared to get a noble gas structure.
Notice that in this picture, the right hand atom's electrons have been shown as crosses. This is called a Lewis Diagram.
This shows a double bond between the oxygen atoms.
So far we have looked at elements that exist as molecules. We can, of course, react non-metals to form compounds. Ammonia, NH3, is one example:
The nitrogen has 5 outer shell electrons. To get its noble gas structure, it needs three more which it shares with three hydrogen atoms.
Grade A
Iodine molecules have two atoms of iodine that each share one outer shell electron to form a single bond. The covalent bond is very strong, but bonding between molecules is quite weak. Iodine is a solid at room temperature, but sublimates into a vapour at 30 oC. Sublimation means going from a solid to a gas without turning into a liquid.
Many non-metal elements and small molecular compounds are gases at room temperature, because the forces between molecules are weak. However, water is a liquid, because it is a polar molecule, having a slightly positive and a slightly negative area in the molecule. Therefore there are stronger attractions between molecules. Just as well, because liquid water is essential for life.