Chemical Bonding

Covalent Bond

A covalent bond is formed by the sharing of valence electrons rather than by transfer, the bonds are formed between non-metal atoms. An example of this is hydrogen. Hydrogen atoms have one outer electron. In the hydrogen molecule each atom contributes one electron to the bond. Therefore each hydrogen atom has control of 2 electrons, one of which is its own and the second electron is from the other atom, giving it the electron configuration of an inert gas i.e. helium. H2O the oxygen atom, with six outer electrons gains control of an extra two electrons supplied by the two hydrogen atoms. Similarly each hydrogen atom gains control of an extra electron from the oxygen.

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Covalently bonded substances fall into two main types: simple molecular substances and giant covalent substances. Simple molecular substances form very strong covalent bonds to form small molecules of several atoms. By contrast the forces of attraction between these molecules are very weak, this gives the substances a very low melting and boiling point this is because although the substances have very strong bonds between the atoms, the intermolecular forces holding the molecules together are weak. The majority of molecular substances are gasses or liquids in room temperature. Examples of simple molecular substances are: oxygen, chlorine and water.

Giant covalent structures are the other main type of covalent bonds where all the atoms are very strongly bonded to each other. Their melting and boiling points are very high in comparison to the simple molecular substances. The atoms are usually arranged into giant regular lattices giving the structures strength because of the many bonds involved. The main examples this structure are diamond and graphite which are both made only from carbon atoms. In diamonds each carbon atom forms four covalent bonds in a very rigid covalent structure and graphite where each carbon atom only forms covalent bonds, creating layers which are free to slide over each other, and leaving free electrons. Graphite is the only non-metal that conducts electricity.    

Intermolecular bonds

Bonds that occur between molecules (intermolecular bonds) are relatively weak and rely on dipole-dipole attractions which may be between permanent dipoles or between temporary/ induced dipoles, the latter are weaker than between permanent dipoles. Temporary or induced dipole forces are generically called Van der Waals forces.  

An example of an intermolecular bonded substance is water. Water has a very high boiling and melting point these characteristic are a result of intermolecular bonding known as hydrogen bonding. A hydrogen bond is formed when hydrogen is covalently bonded to the very electronegative element such as oxygen, fluorine or nitrogen. In water the hydrogen-oxygen covalent bond is very polar. As hydrogen has only one electron when the water pulls the bonded pair of electrons towards itself, it leaves the hydrogen atom’s nucleus of a single proton more exposed. The resulting δ+ on the hydrogen atom is attracted to the lone pair (δ-) of the oxygen atom to form a fairly strong intermolecular attraction.

Metallic bonding 

In metals, the metal atoms lose their outer electrons to form metal cations. The electrons from all the metal atoms form a sea like structure of electrons that can flow around these metal cations. These electrons can be described as delocalised electrons. As the metal cations and the electrons are oppositely charged, they will be attracted to each other, and also to other metal cations. These electrostatic forces are called metallic bonds, and these are what hold the particles together in metals. Since the metal ions in metallic bonds are closely packed together, it is not surprising that most metals have high densities such as stainless steel.  

References:

- www.bbc.co.uk (20/03/08)

-  (20/03/08)

- Edexcell AS Chemistry, by George Facer. October 2004

- Chemistry AS Success by Bob Mcdual. (2003)