What are repulsive interactions?

Intermolecular forces are the repulsive or attractive interaction between the molecules. Due to the coulomb’s law, there will be a coulomb force that exists between two or more charged species. If the charge of both molecules is positive or negative, then that will lead to repulsive forces. When they have opposite charges, then the force is attractive.

Intermolecular forces are the secondary forces that create the interaction between the molecules, which includes attractive & repulsive forces. If the two species experience an intermolecular force, a -ve charge on one species is attracted to the +ve charge on the other species. If a compound has strong intermolecular forces, then the molecules, ions, or atoms are highly attracted to each other whereas if a compound has weak intermolecular forces, the molecules do not have attraction, and they are away from each other.

Intermolecular forces

The intermolecular forces could be categorized as follows:

• Dipole-dipole forces.
• Dispersion forces.
• Van der waals forces.
• Hydrogen bonding.
• Ionic bonding.

Dipole-Dipole forces

Dipole-dipole force is a type of attractive force that occurs between polar molecules. Consider the example, a molecule of hydrogen chloride (HCl) contains a partially -ve chlorine atom & a partially +ve hydrogen atom. A combination of many HCl molecules would arrange themselves and hence the oppositely charged area of the adjacent molecules are close to each other. The molecular dipole moment & the stability of the intermolecular forces are directly proportional to each other. Hence, as the dipole moment increases, the stability of the intermolecular forces also increases. The dipole-dipole forces that exist between HCl molecules could be represented as follows:

Dispersion forces

Among all intermolecular forces, the weakest force is the dispersion force. This is also named as London dispersion forces. These forces are existing between all the atoms & molecules. This force is caused due to the random movement of the electrons.

Consider the example of a helium atom, it has two electrons & when averaged over time, these two electrons would disperse themselves orderly around the nucleus. But at a particular time, the electron dispersion might be irregular or uneven which will lead to an instantaneous dipole. Such temporary or weak dipole could subsequently impact the adjacent helium atoms via electrostatic repulsion or attraction. The production of an induced dipole can be given as follows:

Dispersion forces that exist between the two atoms in uneven molecules could attract the two molecules to each other. Such forces are comparatively weak & essential only if the molecules are adjacent to each other. Heavier & larger molecules and atoms show high dispersion forces than the lighter & smaller atoms& molecules. For example, at room temperature, chlorine and fluorine are gases, which reflects that the attractive forces are weaker and bromine is liquid and iodine is solid which reflects that the attractive forces are stronger. The valence electrons are on average in the case of the larger atom and away from the nuclei than in the smaller atom. Therefore they are loosely bound and could more readily produce the temporary dipoles which create attractions.

A molecule that contains a readily distorted charge cloud is strongly polarizable and would have high dispersion forces. A molecule, which has a charge cloud that is very hard to distort is less polarizable & has small dispersion forces. The power of the dispersion force is essentially based on the shape of the molecule because how one molecule could interact with an adjacent molecule is determined by the shape.

Van der waals forces

The van der Waals interactions are the type of forces that includes repulsion & attraction between molecules, surfaces & atoms. The van der Waals is caused because of the transient change in electron density. Particularly, the electron density can temporarily move to one part of the nucleus. This will produce a transient charge to which a nearby atom could be either repelled or attracted.

Hydrogen bonds

The force which exists between the water molecule is dipole-dipole interaction. In water, the hydrogen atom is connected to strong electronegative oxygen (O) atom that creates the molecule to be polar. The oxygen atom of one water molecule is pulled by the partially +ve hydrogen (H) atom of the adjacent water molecule. The hydrogen bonding could be represented as follows:

H-bonds are weaker than covalent bonds and stronger than the dipole-dipole interactions. H-bonding could occur in the molecule where the H atom is covalently connected to either nitrogen (N), oxygen (O), or fluorine (F). Because the electronegativity of N, O & F are very high and the majority of electron density from the covalent bond is withdrawn by them and make H atom electron deficient.

The water molecule possesses some uncommon but essential properties due to this hydrogen bonding. Consider the example, the compounds which have equivalent molecular mass of water exist as a gas at room temperature but due to this hydrogen bonding, water molecule could be condensed in aqueous.

Ionic bonding

Ionic bonding is an intermolecular interaction between anionic & cationic sites. It is also named salt bridge or ion pairing. It is specifically because of electrostatic forces even though in aqueous solution the attachment is caused by entropy & often even endothermic.

Difference between attractive and repulsive forces

A force between two like or similar charges is named as repulsive force whereas a force between two unlike or dissimilar charges is named as an attractive force.

In the case of repulsive force, the two charges of a like nature are pulled far from each other. For example, the force between two electrons is repulsive, because both the electrons have a negative charge and hence they repel each other. In the case of attractive force, the two charges of unlike nature are attracted each other. For example, the force that exists between an electron and a hole is attraction. Because the electron has a -ve charge whereas the hole has a +ve charge. Therefore, they pull each other.

Context and Applications

This topic is significant for all undergraduates and postgraduates especially for Bachelors and Masters in Chemistry.

Practice Problems

Question 1: London force is also named as________.

1. Covalent bonds
2. Hydrogen bonding
3. Van der Waals forces
4. Dispersion force

Answer: Option 4 is correct.

Explanation: Nonpolar molecules & electrically symmetrical atoms containing zero dipole moment due to their distribution of electrons could create a temporary dipole is also named as London force or dispersion force.

Question 2: Among the following interactions, HCl is an example of which interaction?

1. London interaction
2. Diople-induced dipole interaction
3. Dipole-dipole interaction
4. Van der waals interaction

Answer: Option 3 is correct.

Explanation: For the dipole-diple forces, HCl is the best example. Because the dipole-dipole interactions exist between the molecules containing permanent dipoles.

Question 3: Dipole-dipole interactions are weaker than _______ and stronger than ________ interaction

1. Ion-ion, london
2. London, ion
3. Dispersion, ion-ion
4. London, dipole-induced dipole

Answer: Option 1 is correct.

Explanation: Dipole-dipole forces are weaker when compared to ion-ion interactions and stronger than the London forces. As there is only the involvement of partial charges. Ion-ion forces contain full charges and the London forces contain no charges.

Question 4: In the case of crystals, the covalent molecules are connected by ______.

1. Hydrogen bonds
2. Van der Waals attraction
3. Electrostatic attraction
4. Dipole-dipole attraction

Answer: Option 2 is correct.

Explanation: In the case of crystals, the covalent compounds are connected by Van der Waals interaction because the Van der Waals interactions exist due to transient changes in electron density.

Question 5: The production of an ionic bond is based on the shape of ________.

1. Lattice
2. Atom
3. Molecule
4. Kernal

Answer: Option 1 is correct.

Explanation: The lattice of a crystalline substance is an array of -ve & the +ve ions inside a compound. It is mainly based on the ease of creation of ionic bonds as per lewis & Kossel's ionic bond formation.