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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Physical Properties of Matter > Atomic and Molecular Properties > Intermolecular Forces > Intermolecular Forces > Interactions Between Nonpolar Molecules

Interactions Between Nonpolar Molecules

Nonpolar molecules generally have an equal distribution of electrons, and therefore charge, due to their symmetry about the origin which results in limited interactions between nonpolar molecules. The primary interactions between nonpolar molecules are London Disperson forces. 

Introduction

The interactions between nonpolar molecules are limited to the weakest of intermolecular forces, London Dispersion Interactions, due to the symmetry of the polar bonds of the molecules that result in an even sharing of electrons between different atoms. These London dispersion forces stem from the instantaneous dipoles that occur from random, momentary shifts in charge caused by the constant movement of electrons. The instantaneous dipoles allow the attraction between two nonpolar molecules as a positive instantaneous dipole from one molecule is attracted to an induced negative instantaneous dipole of a second molecule (See Figure 1).

 

instantaneous dipoles.png

Figure 1: Random movement of electrons create an instantaneous dipole that results in a momentary attraction between two nonpolar molecules. Source: Original work.

 

Characteristics of Nonpolar Molecules

Definition: A nonpolar molecule is one with an even distribution of electrons and therefore, no abundance of charges at opposite sides. The charges of the nonpolar molecule balance each other out.Figure 2 Caption.png513px-Methane-CRC-MW-3D-balls.png

Structure

Nonpolar molecules are symmetrical about the center of the molecule. This is to balance the electronegativity of the molecule, as each charge around the central atom must balance in order for there to be no overall polarity to one side of the molecule.

Intramolecular Forces

The bonds in nonpolar molecules are typically covalent, promoting an even sharing and distribution of electrons within a molecule which results in a nonpolar molecule.

Predicting Nonpolarity

  • Diatomic molecules of the same element (i.e. halogens)
  • most carbon compounds (i.e. CO2, CH4, C2H4)

Intermolecular Forces

London Dispersion Forces

  • occur in all substances, but are the most important intermolecular forces in substances that are nonpolar or have no dipole moment
  • due to the movement of electrons, producing an instantaneous (and temporary) dipole that will induce a similar dipole in a neigboring atom and result in a very weak attraction between the two molecules
  • explains physical property trends in halogens, noble gases, and nonmetals as well as in nonpolar molecules
  • general rule of thumb: the greater the number of electrons, the stronger the dispersion forces and the higher the boiling points/melting points of the substance
    • reason: larger molecules have larger, more dispersed electron clouds that lead to increased polarizability and therefore, stronger attraction between atoms (electrons are farther from the nucleus and less tightly held, forming dipoles more easily)
    • i.e. halogens; going down thet halogen family, the diatomic molecules' state at room temperature go from gaseous fluorine and chlroine to liquid bromine to solid iodine. This demonstrates an increase in attraction as the size of the atoms increase down the periodic table. Caption Figure 3.PNG
  • London dispersion forces also increase with an increase in contact between atoms
    • reason: greater surface area contact leads to closer interactions between molecules, which leads to greater attractions

Consequent Effects of London Dispersion Forces

The weak intermolecular forces allow nonpolar molecules to:

  •  condense to liquids
  •  freeze to solids

Problems

1. Is an organic molecule always a nonpolar molecule? 

2. Do intermolecular forces between nonpolar molecules increase or decrease as the size of the molecules become bigger? Why or why not?

3. What kind of forces are present between nonpolar molecules?

4. Why are most noble gases diatomic if their electron shells are already filled?

5. The electronegativity values of molecule A are very similar and the electronegativity values of molecule B differ greatly. Which molecule most likely interacts primarily via instantaneous dipole-induced dipole moments with another molecule of the same type? Why?

Answers.png

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Last Modified
09:32, 2 Oct 2013

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