Intermolecular forces are repulsive or attractive forces that can arise between two molecules due to the action of positively charged protons and negatively charged electrons. Atoms are known to consist of a positively charged nucleus (which is surrounded by electrons that are dispersed over ‘electron clouds’). However, the interaction of a molecule with a charged species can change the distribution of electrons around the nucleus and the shape of the electron cloud that surrounds the nucleus. For example, the electron density may shift towards a specific region in the cloud due to interactions with charged species. The shifts in the electron densities over the electron clouds can result in several repulsive and attractive forces that are called Van der Waals forces. Some important types of intermolecular forces are listed in this article.
Dipole-Dipole Interactions
If the difference in the electronegativities of two bonded atoms in a molecule is very high, the molecule develops a permanent dipole due to the unequal sharing of electrons. Here, the more electronegative atom develops a partial negative charge and the less electronegative atom develops a partial positive charge.
Ion-Dipole Interactions
When polar molecules interact with ions, they are realigned in a manner that minimizes repulsive forces and maximizes attractive forces. For example, when a polar molecule interacts with a positively charged ion, the negatively charged part of the dipole gets attracted to the ion and the positively charged end of the dipole is repelled by the ion.
Dipole-Induced Dipole Interactions
The interactions between a dipole and a neutral molecule can result in a shift in the electron density in the neutral molecule, which is transformed into an induced dipole. Such interactions are called dipole-induced dipole interactions.
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