What is the fundamental difference between intermolecular and intramolecular forces?

The fundamental difference lies in their location and strength. Intramolecular forces are strong attractive forces *within* a molecule, holding atoms together to form the molecule itself (e.g., covalent bonds in $H_2O$). Intermolecular forces are much weaker attractive or repulsive forces that exist *between* separate molecules. Intramolecular forces determine a molecule's chemical identity, while intermolecular forces determine its physical properties and state of matter.

Why are London Dispersion Forces present in all molecules, even nonpolar ones?

London Dispersion Forces (LDFs) arise from the instantaneous, temporary fluctuations in the electron cloud of an atom or molecule. Even in nonpolar molecules, electrons are constantly moving. At any given moment, there might be a slight imbalance in electron distribution, creating a temporary, instantaneous dipole. This temporary dipole can then induce a dipole in a neighboring molecule, leading to a weak, fleeting attraction. Since all atoms and molecules have electrons, LDFs are universally present.

What are the specific conditions required for hydrogen bonding to occur?

Hydrogen bonding is a special, strong type of dipole-dipole interaction. It requires two main conditions: First, a hydrogen atom must be directly covalently bonded to a highly electronegative atom, specifically Fluorine (F), Oxygen (O), or Nitrogen (N). This makes the hydrogen atom very electron-deficient. Second, there must be another highly electronegative atom (F, O, or N) in a neighboring molecule that possesses a lone pair of electrons, which can then attract the partially positive hydrogen atom.

How do intermolecular forces affect the boiling point of a substance?

The boiling point of a substance is the temperature at which its vapor pressure equals the external atmospheric pressure. To boil, molecules in the liquid phase must gain enough kinetic energy to overcome the attractive intermolecular forces holding them together and escape into the gas phase. Therefore, substances with stronger intermolecular forces require more energy (higher temperature) to achieve this, resulting in higher boiling points. Conversely, weaker IMFs lead to lower boiling points.

Why does water have an unusually high boiling point compared to other hydrides like $H_2S$?

Water ($H_2O$) exhibits extensive hydrogen bonding due to the high electronegativity of oxygen and the presence of two lone pairs on the oxygen atom, allowing each water molecule to form up to four hydrogen bonds with neighboring molecules. In contrast, $H_2S$ does not form hydrogen bonds because sulfur is less electronegative than oxygen, making the S-H bond less polar and the hydrogen atom less electron-deficient. The strong hydrogen bonds in water require significantly more energy to overcome during boiling, leading to its anomalously high boiling point.

Intermolecular Forces

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

Intermolecular forces (IMFs) are the attractive or repulsive forces that arise between molecules. These forces are significantly weaker than the intramolecular forces (covalent, ionic, or metallic bonds) that hold atoms together within a molecule. IMFs are responsible for many of the macroscopic physical properties of substances, such as melting points, boiling points, viscosity, surface tension, …

Quick Summary

Intermolecular forces (IMFs) are attractive or repulsive forces between molecules, distinct from the much stronger intramolecular forces (chemical bonds) within molecules. They dictate a substance's physical properties like melting point, boiling point, viscosity, and solubility.

The main types of IMFs include Van der Waals forces and hydrogen bonding. Van der Waals forces comprise London Dispersion Forces (LDFs), which are universal and arise from temporary electron cloud fluctuations, increasing with molecular size and surface area.

Dipole-Dipole forces occur between polar molecules with permanent dipoles. Dipole-Induced Dipole forces involve a polar molecule inducing a temporary dipole in a nonpolar one. Hydrogen bonding is a particularly strong dipole-dipole interaction occurring when hydrogen is bonded to F, O, or N, and attracted to another F, O, or N.

The general order of strength is Hydrogen bonding > Dipole-Dipole > LDFs. Stronger IMFs lead to higher boiling points, melting points, viscosity, and surface tension, and lower vapor pressure. This understanding is crucial for explaining the states of matter and their transitions.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

London Dispersion Forces (LDFs)

LDFs are the most fundamental type of intermolecular force, present in all substances, regardless of their…

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of intermolecular attraction, crucial for many biological and…

Impact on Physical Properties (Boiling Point)

The strength of intermolecular forces directly dictates the amount of energy required to separate molecules…

IMFs vs. Intramolecular: — IMFs are *between* molecules (weaker, physical properties); Intramolecular are *within* molecules (stronger, chemical properties).

Types of IMFs (Weakest to Strongest):

1. London Dispersion Forces (LDFs): All molecules. Instantaneous dipoles. Strength $propto$ Molecular size/Polarizability. 2. Dipole-Dipole Forces: Polar molecules. Permanent dipoles. 3. Hydrogen Bonding: H bonded to F, O, or N, attracted to another F, O, or N. Strongest IMF.

Physical Properties & IMFs:

* Stronger IMFs $ightarrow$ Higher Boiling Point, Higher Melting Point, Higher Viscosity, Higher Surface Tension. * Stronger IMFs $ightarrow$ Lower Vapor Pressure.

Anomalies: —

H_2O

NH_3

HF

have unusually high boiling points due to hydrogen bonding.

Hydrogen Doesn't Like Dancing In London.

Hydrogen Doesn't: Hydrogen Dipole (Hydrogen Bonding)

Like Dancing: London Dispersion (LDFs)

In London: Induced London (Induced Dipole-Induced Dipole)

(This mnemonic helps recall the types, though it doesn't strictly follow strength order. For strength, remember H-bond is strongest, then Dipole-Dipole, then LDFs.)