Bonding

There are three types of bonding, ionic, covalent and metallic bonding.

Ionic bonding is based on electrostatic attraction between ions, charged atoms that have either lost or picked up one or more electrons.

Ionic bonding involves the transfer of electrons. It happens between metals and non-metals, between which there is a large difference in electronegativity. Metals lose electrons and become positively charged, whereas non-metals pick up electrons and become negatively charged (remember that electrons are negatively charged sub-atomic particles). The main driving force for this is the formation of a full outer shell, which is the most stable arrangement. Metals have only a few outer electrons, so for example losing three outer electrons in aluminium is energetically easier than picking up 5 extra electrons to obtain a full shell. For non-metals the situation is different in that they only need a few more electrons to get to a full outer shell, e.g. chlorine has 7 outer electrons and so only needs 1 more electron for a full outer shell.

Ions in an ionic compound arrange themselves in giant ionic structures with the positive ions surrounded by the correct ratio of negative ions, and vice versa.

Covalent bonds involve the sharing of electrons between two atoms. This type of bond forms between non-metals, between which there is not a great difference in electronegativity. Again, the main driving force is stability and a low-energy arrangement, and again the most favourable arrangement is a full outer shell (there are exceptions to this, such as boron and phosphorus). For example, carbon has four outer electrons and would have to either lose or pick up 4 electrons to form ionic compounds. For elements like carbon it is easier to share electrons with other elements. Each covalent bond is the sharing of two electrons. So for the carbon compound methane, CH₄, carbon shares one of its 4 outer electrons with one hydrogen atom each, and each hydrogen atom shares its outer electron with carbon. Effectively each atom now feels as if it has a full outer shell: carbon feels as if it has 8 outer electrons, and hydrogen as if it has 2 outer electrons (hydrogen fills the first shell, which can only hold a maximum of 2 electrons, whereas the second and third shell can hold up to 8 electrons).

Covalent structures can exist as giant covalent structures (e.g. diamond or graphene) or as simple molecules (e.g. hydrogen or oxygen).

Metallic bonds are formed between metals. In a metal the outer electrons are less tightly attracted to the nucleus than in other elements, so they are easily lost to form a sea of delocalised electrons , leaving the positively charged metal ions arranged in a regular giant lattice. The electrostatic attraction between the metal ions and the delocalised electrons glues the structure together. Because of the freely moving delocalised electrons, metals are good conductors of electricity and heat.