Definition of Intermolecular Forces
The attractive forces exist between individual particles i.e. atoms, molecules, and ions.
It is very important to realize that the attraction between the particles is much weaker than the attraction between atoms within a molecule. In a molecule of HCl, there is a covalent bond between H and Cl which is because of the mutual sharing of electrons. Both atoms satisfy their outermost shells and it is their firm needs to stay together, hence this bond is very strong.
HCl molecules in the nearby attract each other, but the forces of attraction are weak. These forces are thought to exist between all types of atoms and molecules when they are sufficiently near each other. Such intermolecular forces are called van der Waals forces and they have nothing to do with the valence electrons.
These intermolecular forces bring the particles close together and give specific physical properties to the substances in gaseous, liquid, and solid states.
Types of Intermolecular Forces
The 4 types of such forces are discussed here:
- Dipole-dipole forces
- Ion-dipole forces
- Dipole-induced dipole forces
- Instantaneous dipole-induced dipole forces or London dispersion forces
“The positive end of one molecule attracts the negative end of the other particle and these electrostatic forces of attraction are called dipole-dipole forces.”
In the case of the HCl molecule, both atoms differ in electronegativity. Chlorine being more electronegative develops the partial negative charge and hydrogen develops the partial positive charge. So, whenever the particles are close to each other, they tend to line up. Thus, attract the opposite ends. However, thermal energy causes the particles not to have a perfect alignment.
Anyways, there is a net attraction between the polar molecules. These forces are called dipole-dipole forces and they are roughly one percent as effective as a covalent bond. The strength of these forces depends upon the electronegativity difference between the bonded atoms and the distance between the molecules.
The distance between molecules in the gaseous phase is greater so these forces are extremely weak in this phase. In liquids, these forces are fairly strong.
The greater the strength of these dipole-dipole forces, the higher are the values of thermodynamic parameters like melting points, boiling points, the heat of vaporization, and heats of sublimation.
The examples of the particles which reveal dipole-dipole attractions are numerous. Two of these are HCl and CHCl3 (chloroform).
An ion-dipole force is an attractive force that arises from the electrostatic attraction in between an ion and a neutral particle that has a dipole. Most typically present in solutions. A positive ion (cation) draws in the partially negative end of a neutral polar molecule.
Dipole-induced Dipole Forces
In some cases, we have a mixture of substances including polar and non-polar molecules. The positive end of the polar particle attracts the mobile electrons of the neighboring non-polar molecule. In this way, polarity is caused in the non-polar molecules, and both molecules become dipoles. These forces are called dipole-induced dipole forces or Debye forces.
Instantaneous Dipole-induced Dipole Forces or London Dispersion Forces
The momentary force of attraction created in between the instant dipole and the induced dipole is called immediate dipole induced dipole interaction or London forces.
The forces of attraction present amongst the non-polar molecules like helium, neon, argon, chlorine, and methane require unique attention due to the fact that under typical conditions such particles do not have dipoles. We know that helium gas can be liquified under proper conditions. In other words, forces of attraction run among the atoms of helium which trigger them to cling together in the liquid state.
- A German physicist Fritz London in 1930 offered a simple description of these weak appealing forces in between non-polar particles.
In helium gas, the electrons of one atom influence the moving electrons of the other atom. Electrons fend off each other and they tend to stay as far apart -as possible. When the electrons of one atom come close to the electron of another atom, they are pushed far from each other. In this way, a short-term dipole is produced in the atom.
The result is that, at any moment, the electron density of the atom disappears balanced. It has a more negative charge on one side than on the other. At that particular moment, the helium atom becomes a dipole. This is called an immediate dipole.
This rapid dipole then disrupts the electronic cloud of the other close-by atom. So, a dipole is induced in the 2nd atom. This is called an induced dipole.
It is a very short-term attraction since the electrons keep moving. This motion of electrons causes the dipoles to disappear as rapidly as they are formed. Anyhow, a minute later on, the dipoles will appear in a different orientation,s and once again weak attractions are established.
London forces exist in all kinds of particles whether polar or non-polar, but they are very significant for non-polar particles like Cl2, H2, and noble gases (helium, neon, etc.).
Factors Impacting the London Forces
- London forces are weaker than dipole-dipole interactions. The strength of these forces depends upon the size of the electronic cloud of the atom or molecules. When the size of the atom or molecule is big then the dispersion becomes easy and these forces end up being more prominent. The elements of the zero group in the periodic table are all mono-atomic gases. They don’t make covalent bonds with other atoms since their outermost shells are complete. Their boiling points increase down the group from helium to radon.
- The atomic number increases down the group and the outer electrons move away from the nuclei. The dispersion of the electronic clouds ends up being increasingly easier. So, the polarizability of these atoms goes on increasing.
- Polarizability is the quantitative measurement of the degree to which the electron cloud can be polarized or distorted. When we say that a species (atom, molecule, or ion) is polarized, it suggests that short-term poles are developed. This is possible if the electronic cloud can be interrupted or distorted. This increased distortion of the electronic cloud produces stronger London forces and for this reason, the boiling points are increased down the group.
- Similarly, the boiling points of halogens in group VII-An also increase from fluorine to iodine. All the halogens are nonpolar diatomic molecules, but there is a huge difference in their physical states at room temperature. Fluorine is a gas and boils at -188.1 ° C, while iodine is a solid at room temperature which boils at +184.4 ° C. The polarizability of the iodine molecules is much greater than that of fluorine.
- Another essential factor that affects the strength of London forces is the number of atoms in a non-polar molecule. The greater the number of atoms in a molecule, the higher is its polarizability. Let us talk about the boiling points of saturated hydrocarbons. These hydrocarbons have a chain of carbon atoms linked with hydrogen atoms. Compare the length of the chain for C2H6 and C6H14. They have the boiling points – 88.6 ° C and 68.7 °C, respectively. This means that the molecule with a large chain length experiences a stronger attractive force. The factor is that longer molecules have more places along its length where they can be attracted to other particles. It is very intriguing to understand that with the increasing molecular mass of these hydrocarbons, they change from gaseous to liquid and after that finally end up being solids.