Intermolecular Forces and Chemical Structures

Covalent Character in ionic bonds
Polar covalent bond – This is when bonding pairs of electrons are shared unequally between atoms that have a big difference in the electronegativity resulting in a permanent dipole

Ionic bonding can display covalent bonding characteristics when an electron cloud distortion forms. This is when a cloud of electrons are draw closer towards the positive ion. This occurs when the cation is small with a high charge density and the anion is large with a lower charge density.

Berillium Chloride

Polarised anions have a changed shape than non-polarised anions due to the electron cloud distortion

Intermolecular Forces

Van der Waals forces
The random movement of electrons creates a temporary fluctuating dipole, which induces a dipole on other molecules. Van der Waals even occur in the Nobel gases.

The larger the molecule – the more electrons – the stronger the Van der Waals forces. Therefore the larger the molecule the greater the energy required to break the bonds, which means the molecule has a higher boiling point. A high boiling point means the element is less volatile (volatile – how easily an element evaporates).

Element Boiling point (°C)
F2 -188
Cl2 -35

Van der Waals forces are the weakest of the three intermolecular forces.

Permanent dipole-dipole intermolecular forces
Compounds with polar covalent bonds, which are formed due to a large difference in electronegativity resulting in a permanent dipole, have polar covalent bonds – these are stronger than Van der Waals forces, which results in a higher boiling point.

Permanent dipole-dipole forces

Hydrogen Bonding

Hydrogen bonding occurs between a hydrogen atom attached to a highly electronegative atom (nitrogen, oxygen or fluorine) and the lone pair of electrons in a highly electronegative atom in the next molecule.

In the exam when drawing a diagram of a hydrogen bond you must include the dipoles, the molecules and a hydrogen bond (represented by a dashed line) between the hydrogen atom and the lone pair of electrons in an atom in the next molecule.

Odd nature of water
Because water has hydrogen bonds it has some unusual properties:
· It has a higher boiling point than expected due to the strong hydrogen bonds.
· Less dense than water when frozen because the hydrogen bonds form permanently resulting in a 3D structure with the molecules less closely packed together than water.

Chemical bonds
The bonds between elements atoms vary between metallic bonding, simple covalent bonding and giant covalent bonding.

Metallic bonding
Definition: the electronstatic force of attraction between the cations of metal (positive ions) and the sea of negatively charged delocalised electrons.


· High melting and boiling points due to the strong electrostatic force of attraction between the cations and electrons.
· Good conductors of electricity due to the sea of moving electrons
· Malleable and ductile because the layers of metal atoms can slide past each other without breaking the bond.

Giant covalent bonds
These occur in atoms with four outer electrons. Therefore carbon (in the form of diamond and graphite) and silicon can form giant covalent bonds.

In diamond and silicon they both form 4 covalent bonds, which forms a tetrahedral shape with a bond angle of 109.5°. As all the outer electrons form a covalent bond the structure has no weak Van der Waals forces so diamond and silicon are very hard and have a high melting/boiling point. Because there are no free electrons in diamond or silicon they both cannot conduct electricity.

Graphite is another form of carbon. It has a triginal arrangement with three covalent bonds in layers. In graphite there are weak Van der Waals forces between layers which is why graphite is soft. The fourth electron is delocalised and therefore graphite (unlike other non-metals) can conduct electricity.

All giant covalent structures have high melting and boiling points due to the strong covalent bonds.