Covalent bonding as you learned in your gcse chemistry classes involves the sharing of a pair of electrons. The atoms involved in a covalent bond are held together by the electrostatic attraction of the positively charged nuclei and the negative charge on the shared pair of electrons. Covalent bonding occurs between non-metal atoms.
Reactive non-metals are mostly found in groups 5, 6 and 7, this means
that their outer electron shells are almost full, so when these react they will gain
electrons, this results in them having
a p6 electron configuration in their outer electron sub-shell.
Most covalent substances consist of small molecules (a molecule is simply a small group of atoms). Covalent substances are often described as having a molecular structure. Ammonia is a covalent susbstance you first met in gcse chemistry, it is a small molecule made up of only 4 atoms, one atom of nitrogen and 3 atoms of hydrogen. Its chemical formula is NH3. It is a covalent compound since it contains only non-metals.
Ammonia is a small covalent molecule, the ammonia molecule has 3N-H covalent bonds and a lone pair or non-bonding pair of electrons. The image opposite shows an ammonia molecule with its lone pair of electrons.
Below is a dot and cross diagram to show how each of the covalent bonds in ammonia are formed. A nitrogen atom, electron arrangement 2,5 is shown. Its outer
shell electrons are shown as green dots, recall from gcse chemistry that in dot and cross diagrams we only show the
outer shell or valency electrons, since these are the ones involved in bonding. Hydrogen is also shown with its one outer shell
electron, shown as a black cross, X.
In order to achieve full last shells both these non-metal atoms share electrons. The electrons are shared in pairs. A pair of shared electrons results in a covalent bond being formed between the two atoms. The nitrogen atom forms 3 covalent bonds, or shares 3 electrons with hydrogen atoms, it does this to achieve a full last shell of electrons. Hydrogen only needs to gain 1 electron to fill its outer electron shell so each hydrogen atom will only form 1 covalent bond.
Methane is the gas we use to heat our homes and for cooking with, it is also the gas used in Bunsen burners in the science lab. Its chemical formula is CH4, it is a covalent compound consisting of 1 atom of carbon and 4 atoms of hydrogen. Carbon has an electron arrangement of 2,4 or 1s22s22p2. So each carbon atom needs to gain 4 electrons to completely fill its outer shell, so it will make 4 covalent bonds. In the example below you can clearly see that each carbon atom bonds with 4 hydrogen atoms, the final diagram shows a dot and cross diagram for the methane molecule, remember that a covalent bond involves the sharing of 2 electrons.
Oxygen is a group 6 element with an electron arrangement of 2,6 or 1s22s22p4. So it needs to gain 2 electrons to obtain a full outer electron shell and as before hydrogen only needs to gain one electron to fill its last shell. To gain 2 electrons each oxygen atom will make 2 covalent bonds, while each hydrogen only needs to gain 1 electron, so it will make one covalent bond. Every atom in the water molecule ends up with a full last shell of electrons. The oxygen atom also has 4 electrons in its last shell which are not used in bonding, these will form the 2 lone pairs as shown.
If atoms in a molecule share only 2 electrons then they will form single covalent bonds. However if atoms have to share more than 2 electrons in order to achieve full last electron shells then double or even triple covalent bonds will be formed, e.g. study the dot and cross diagrams below for hydrogen, oxygen and nitrogen molecules. Oxygen atoms have 6 electrons in their outer shell so need to gain two electrons, so when oxygen atoms join to form oxygen molecules these molecules will contain double covalent bonds between the atoms. Nitrogen atoms having only 5 electrons in their outer electron shells need to gain 3 more electrons to complete the outer electron shell, so when nitrogen atoms join there will be six electrons in the area of overlap between the atoms, this means that there is a triple covalent bond between the atoms. The image below explains this in more detail:
Carbon dioxide (CO2) is small covalent molecule that contains double covalent bonds between the atoms of carbon and oxygen. Carbon, electron arrangement 2,4 needs to gain 4 electrons to complete its octet of electrons and oxygen; electron arrangement 2,6 needs to gain 2 electrons to fill its last shell. In order for both the carbon and oxygen atoms to end up with full last shells they share 4 electrons between them. This results in a double C=O covalent bond forming. The diagram below shows a dot and cross diagram for carbon dioxide.