Types of Bonds
All bonds, in and around an atom, fall into two categories – Intermolecular and Intramolecular bonds. These two categories can then be broken further down to three subcategories;
Chemical Bonds – A chemical bond is a link between atoms via the sharing of electrons through overlapping orbitals. A typical chemical bond will contain two electrons and one electron will usually originate from each of the atoms involved. The electrons shared need not necessarily originate from both of the atoms and neither must the number of atoms involved be two, 3-centred two electron bonds exist.
This type of bond is the strongest of all three bonds and thus requires the most energy input to break - ranging from 151 KJ/mol (I-I) to 945 KJ/mol (N≡N). There are two types of chemical bonds; Ionic and covalent bonds.
Van-Der-Waals – This type of bond is the weakest of the three bonds and originates from the fact that electrons are constantly in movement. Because they do not all move in unison, ‘wells’ where there is a net negative and positive charge appear. These opposite charges then attract each other and albeit weakly congregate molecules together. As these bonds rely on electrons, the greater the number of electrons in a molecule – and thus the size of the molecules themselves – the greater these forces are. These forces may then be split into two sub categories; London Dispersive forces and dipole-dipole interactions.
Hydrogen Bonds – These bonds are a strong intermolecular forces, much greater than Van-Der-Waals and yet is relatively small in comparison to a chemical bond. A hydrogen bond can range from about 5-35 KJ/mol. ‘Clusters of Molecules are held together in specific orientations and often in well-defined numbers; the equilibria among the clusters and their component molecular parts are rapid and reversible’1.These forces though commonly intermolecular, are not necessarily so – we will come to this later on.
What is a Hydrogen Bond?
‘A hydrogen bond is a non-covalent, attractive interaction between a proton donor X-H and a proton acceptor Y in the same or different molecule:
X-H- - -Y’4
According to convention, a hydrogen bond is formed when a H atom
is bound to an electronegative atom such as N, O, or F - this bond being the X-H in the above description - forms a weak bond with a polar molecule. The reason this occurs is because the electronegative atom that hydrogen is bound to pulls the electron density away from the hydrogen. As we can see in Figure 1; This in turn causes the hydrogen that is left bare - and thus slightly electron deficient - to acquire a δ+ charge (the delta indicates that the charge is small) and the ‘X’ atom to acquire a δ- charge due to an increase of electron density. These two charges then attract intermolecularly.
However, ‘the experimental and theoretical results reveal that even C-H can be involved in H-bonds and π electrons can act as proton...