Intermolecular Forces

Dipole-dipole interactions occur in polar molecules where the difference in electronegativity between the combining atoms creates positive and negative dipoles. These opposite poles align and result in electrostatic attraction throughout the polar medium. So, naturally, the strength of the interaction would depend on the magnitude of the charges and their distance, explained by the Coulombic law. So, the higher the magnitude of the charges and the lesser the distance between them, the stronger the dipole-dipole attractive interaction.

Among the various types of solids—metallic, ionic, and covalent—the molecular solid stands out for its intermolecular attractive strength.

A molecular solid is composed of distinct, unconnected molecules that are held together by weak van der Waals forces or Hydrogen bonds rather than stronger ionic or covalent bonds. 

Van der Waal’s interactions are the weak intermolecular forces of interactions. The Van der Waal’s forces are comprised of three forces. Their different names and the order of their strength is-

Dipole-Dipole > Dipole-induced dipole > Induced Dipole-Induced Dipole

Debye intermolecular forces of attraction occur between polar and nonpolar atoms or molecules.

When a polar molecule is in the vicinity of a nonpolar molecule, the electronegative end of that polar molecule attracts the electron cloud of the adjacent nonpolar molecule, thereby distorting it and inducing a pole separation. So, the polar molecule is said to have induced polarity in the neighboring nonpolar molecule.

Debye intermolecular forces of attraction occur between polar and nonpolar atoms or molecules. When closely placed with a polar molecule (electronegativity difference between 0.5-1.7), the electron cloud of the nonpolar atom (or molecule) is influenced and distorted, leading to the formation of temporary dipoles. As the polarity is induced (forced) in the otherwise nonpolar atom, the Debye forces are said to show an Induction effect.

If the size of a nonpolar atom is greater, it means that the atom has a higher number of electron containing orbitals. The outer electrons are away from experiencing the inward attractive pull of the nucleus (nuclear charge) and capable of undergoing electron-cloud distortion or polarization. 

Debye interactions require polar molecules to show polar-nonpolar type of intermolecular attractive interactions. Therefore, the polar atoms participating in Debye interactions must have comparatively high electronegativity to show stronger interactions. If the electronegativity difference (0.5-1.7) is greater in a polar molecule, the polarity it induces on the neighboring non-polar atom will also be more significant, leading to stronger Debye interactions.

The strength of polar-nonpolar interactions is affected by the atomic size of the nonpolar atom (or molecule) and the electronegativity of the polar molecule. 

A polar molecule has natural poles. Therefore, when next to a nonpolar molecule, it will try to magnetize it temporarily. 

The negative end of the polar molecule will repel the nonpolar molecules’ moving electron cloud, or the positive end will attract its electrons, which can contribute to significant distortions. Such a forced activity can induce a temporary pole separation in the nonpolar molecule. 

London dispersion forces are the weak intermolecular attractive forces that occur when the molecules are spaced closely. If the spacing is compact, the interactions are stronger. This means that the temporary dipoles induced due to the dispersion of electrons are also larger. Such a compact cluster will require more energy to break, raising the physical properties of the melting and boiling point.

London dispersion forces are the weak intermolecular forces that occur when the molecules are in close proximity to each other. The strength of these forces is determined by the atomic size of the element.

In smaller atoms, the atomic radius is smaller, which means that the positively charged nucleus holds the outermost valence electrons more firmly, leaving less room for their dispersion or polarization.

London dispersion forces are weak intermolecular forces found in all atoms and molecules. However, it is an exclusive force binding the nonpolar molecules due to their lack of functional groups, avoiding competing interactions.

London Dispersion forces are seen in all the atoms and the molecules; however, it is the exclusive force binding nonpolar molecules. Nonpolar molecules are made of two or more atoms, and the electronegativity between two atoms in a covalent bond is less than 0.5. Therefore, no charges are separated to create poles in nonpolar molecules or atoms. Despite not having poles, nonpolar molecules interact using a weak force of attraction known as the London Forces.

All atoms and molecules show London Dispersion Force, a primary way atoms and molecules interact. Most of the time, molecules have additional stronger forces than the London Dispersion Force, which is the weakest. Therefore, London Dispersion forces are exclusively seen in the nonpolar molecules since they don't have other competing functional groups leading to other interfering intermolecular interactions.

A nonpolar molecule is made of two or more atoms where the electronegativity difference between the atoms is less than 0.5. Therefore, the electron density in the bond is equally distributed, and there is no concentration of electrons on one atom to create a charge separation rendering the molecule essentially without poles or nonpolar. Such nonpolar molecules like H₂, CH₄, He, etc., will interact using the London dispersion forces without forming any positive or negative pole.

London dispersion force is the weakest of the three van Der Waal forces of interactions universally found in all the atoms and molecules. It is caused by the constant fluctuations in the electron position, creating a temporary dipole. When other nonpolar molecules are in its vicinity, the instantaneous temporary dipole formed earlier distorts the electron cloud of an adjacent molecule, creating another dipole.

London dispersion forces are the weakest intermolecular forces among all the van der Waal forces of interactions. This interaction occurs in molecules due to electrons' continuous random motion, creating a temporary dipole. The instantaneous dipole in one molecule induces a dipole in the others, causing dispersion. The molecules then start to show intermolecular attraction.

Hydrogen bonding is a type of electrostatic interaction occurring in molecules in which the H atom is sandwiched between highly electronegative atoms like F, O, and N, one to which it is covalently bonded. For example, HF, H₂O, NH₃, etc.

Keesom forces occur in polar molecules where the atoms have a considerable electronegativity difference, resulting in the charge separation and formation of positive and negative poles. The negative pole of one molecule electrostatically interacts with the positive pole of the neighboring molecule. Consequently, all the polar molecules in the medium try to orient their dipoles to be in alignment for such electrostatic interactions.

Polar molecules show Keesom forces. Polar molecules have heteroatoms that differ in electronegativity values such that the electronegativity difference between the two atoms in a polar covalent bond is greater than 0.5 but less than 1.7. For example, HF, HCl, R-OH, etc.

A molecule is said to be polar if the bond electrons between the atoms in a covalent bond are unequally distributed, creating two ends. The electron redistribution is marked by showing the separation of charges as a partial charge above the atom's symbol. Therefore, a polar molecule always has a positive and a negative end resulting in a dipole. The pulling of bond electrons by an atom is attributed to its electronegativity.

There are three types of Van der Waal’s forces: Keesom Forces, Debye forces, and London Dispersion forces.

The van der Waal forces of interactions are weak intermolecular forces. For the atoms or molecules show these interactions only under certain conditions -

1) Distance- van der Waal forces mainly depend on the distance between the associated molecules. These forces become very weak if the distance between the molecules or atoms is greater. These forces are relatively stronger if the atoms or molecules are grouped closely.

There are three types of Van der Waal’s force- Keesom Forces, Debye forces, and London Dispersion forces. Amongst the three types, the London Dispersion forces are the weakest interactions.

Van der Waals forces are weak intermolecular attractive forces that occur in polar and nonpolar atoms or molecules due to the shift in their electron positions.

The electrons shift to form electron-dense and electron-deficient poles. Some poles are permanent due to the nature of the atom in a molecule, while others are induced poles. 

Van Der Waals forces are weak intermolecular attractive forces that hold closely various atoms and molecules without any physical linkage or bond. 

The atoms or molecules participating in van Der Waals interactions are neutral, polar, or /and non-polar ones. Ion interactions are stronger and are classified separately from the weaker van Der Waal interactions.

Water at room temperature is liquid in nature due to the hydrogen bonds holding the water molecules (H₂O). The close association is hard to break; therefore, water boils at a high temperature of 100^oC. The high temperature breaks the Hydrogen bond links to free individual H₂O molecules.