Forces Between Covalent Molecules

Molecules are formed due to the reaction of atoms of either the same or different compounds. The primary cause of these reactions is the release or absorption of energy or achieving noble gas stable configuration. But when a molecule has been formed then its atoms are held together by the nuclear force. So if atoms in a molecule are held together by nuclear force then how different molecules are held together.

A glass of water has millions of H2O molecules, how these molecules are held together? Well, these molecules are held together via Intermolecular Forces, a force between different molecules.

Here in this article, I will discuss different types of Intermolecular Forces.

What are Intermolecular Forces?

Forces that exist between molecules are called Intermolecular Forces, these forces held different molecules of one compound or of multiple compounds(in case of a mixture) together.

Depending upon the nature of molecules there can be different types of Intermolecular Forces amongst them. Some of common types of Intermolecular Forces are as following: –

Type of MoleculeIntermolecular Force
Molecules having Dipoles• Dipole-Dipole Attractions
• Hydrogen Bonding
• London Dispersion Force
Molecules not having DipolesLondon Dispersion Force only

For example – Hydrogen Chloride HCl molecules have dipoles because the electronegativity of Chlorine is way higher than that of hydrogen. As HCl molecules have positive/negative dipoles that’s why there exists Dipole-Dipole attraction between different molecules of HCl, which hold different molecules together.

What are Dipole-Dipole Attractions?

Dipole-Dipole Attractions is the force that exist between polar molecules. As Polar Molecules(like HCl, H2O) have two poles(one positive and one negative) on opposite ends of the molecule, that’s why due to closure proximity of positive pole of one molecule to negative pole of another molecule, there exists an Electrostatic Force which is called Dipole-Dipole Attractions.

What is Hydrogen Bonding?

Hydrogen Bonding is a special type of Dipole-Dipole attraction in which hydrogen atom acts as a bridge between two electronegative atoms.

Hydrogen Bonding in Water Molecule

What is London Dispersion Force?

Though non-polar molecules like Iodine(I2), Hydrogen(H2) doesn’t have any poles, but as electrons are moving within these molecules that’s why at some particular instant it can be possible that one end of molecule have more electrons as compared to other end, this creates more concentration of negative charge on one side of molecule as compared to other, creating Temporary Dipoles. Which like usual Dipoles can be attracted to each other via Electrostatic Force. This force between molecules created due to Temporary Dipoles is called London Dispersion Force.

Affect of Intermolecular Forces on Properties

Intermolecular Forces largely impact Physical Properties of the compounds. For example – Due to more Dipole-Dipole Attraction between molecules Boiling Point of a compound can be more as compared to other compound which don’t have Dipole-Dipole Attraction.

Types of Compounds which have Dipole-Dipole Attraction

Usually the compounds whose molecules are made up of Electronegative Atoms have Dipole-Dipole Attraction. For example – Most of compounds made by elements of Group 16, 17 with elements from Group 1, 2 in Periodic Table have Dipole-Dipole Attraction. Because difference between electronegativity of Group 16, 17 element and Group 1, 2 is quite huge.

Some Examples of Compounds which have Dipole-Dipole Attraction.

  • Hydrogen Chloride HCl
  • Hydrogen Bromide HBr
  • Water H2O
  • Hydrogen Sulphide H2S

Types of Compounds which have Hydrogen Bonding

Compounds whose molecules have more than one hydrogen atom bonded to a Highly Electronegative Central atom have Hydrogen Bonding. Some examples of compounds which have Hydrogen Bonding are – Water, Ammonia, Alcohols. All three of these have multiple hydrogen atoms linked to Central Highly Electronegative atom.

Types of Compounds which have London Disperison Force

Compounds whose molecules have large atoms and high number of electrons in outershells usually have London Dispersion Force. For example – Iodine molecule(I2) have London Dispersion Force because Iodine atom is quite large(electrons are less attracted towards nucleus) and have many electrons in the outermost shell(more repulsion between electrons), both these factors make it easier for electrons to concentrate onto one side of molecule and then disperse off(due to excessive repulsion) and then again concentrate/disperse. Due to this process, Temporary Dipoles pop up on the molecules and hence causing London Dispersion Force.

Is Intermolecular Forces stronger than Covalent Bond or Ionic Bond?

While Covalent Bond or Ionic Bond is formed by sharing or transfer of electrons between atoms, Intermolecular Forces are caused by just nature of molecules. So relatively Intermolecular Forces are way less stronger as compared to Covalent Bond or Ionic Bond.
Below is a table showing Bond Dissociation Energies required for breaking Covalent or Ionic Bond as compared to an Intermolecular Force. You can clearly see that Energy required for braking Intermolecular Force is almost 100 times less as compared to that required for breaking Covalent or Ionic Bond.

ForceApprox. Energy required for Dissociation(kJ per mol)
Ionic Bond between Na+ and Cl500 to 4000
Covalent Bond between Cl and Cl200 to 1100
Dipole-Dipole Attraction between H – Br and H – Br5 to 25
London Dispersion Force between I2 and Argon molecules40

Relative Strength of London Forces and Hydrogen Bonds

Theoritically it seems that Hydrogen Bonds would be stronger as compared to London Force, because Hydrogen Bond is essentially an Electrostatic Force. But that’s not always the case, sometimes London Forces can be stronger as compared to Hydrogen Bond. For example – Cooking Oil contains molecules which have higher molecular masses and therefore many electrons. The London Dispersion Forces between these molecules are so large that oil has higher boiling point as compared to water.

So it cannot be clearly said whether London Force is stronger or Hydrogen Bond is stronger.

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