Intermolecular Forces - Ahmad Bilal

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Intermolecular Forces

The set of attractive and repulsive forces that occur between molecules

They strength of the intermolecular forces determines the following physical properties of molecular compounds:

Solubility in various solvents

Hardness and Texture

Surface Tension

Boiling Point

Melting Point

Physical state of a compound at a specific temperature and pressure

London Dispersion

In this example, the uneven distribution of electrons in the He atom (left) causes a temporary dipole (middle). This induces a dipole in the neighboring He atom (right).

The force of attraction acting between all entities, including polar molecules, non-polar molecules and unbonded atoms, caused by the temporary imbalance of electrons within the entities

Larger the molecule, the greater the polarizability

The greater the polarizability, the stronger the dispersion forces

Weakest intermolecular force

Temporary dipole induces a dipole into neighboring molecules

Hydrogen Bonding

In this example, the partially negative oxygen atom in one of the water molecules is attracted to the partially positive hydrogen atom in another nearby water molecule

The force of attraction between a hydrogen atom attached to a highly electronegative atom (N, O, or F) and a highly electronegative atom in another molecule

Important in the organization of biological molecules

Stronger than dipole-dipole or dispersion forces

Positive charge is attracted to the negative charge of an electronegative atom in a nearby molecule

Dipole-dipole interaction

Induced Dipole-Dipole

In this example, the water molecule (polar) induces a dipole into the oxygen molecule (non-polar) which results in a weak attraction between the two.

The force of attraction that forms when a polar molecule induces a dipole in a non-polar molecule by disturbing the arrangement of electrons

A weak form of attraction

Dipole-Dipole

If the dipoles of a molecule cancel each other out, then the molecule is non-polar and symmetrical. If not, then the molecule is polar and asymmetrical

In this example, the partial negative charge of the chlorine atom attracts to the partial positive charge of the hydrogen atom of the other compound.

The force of attraction between the slightly positive end of one polar molecule and the slightly negative end of an adjacent polar molecule

Only has a significant effect when the molecules involved are close together

Increase with an increase in the polarity of the molecule

Weaker than ion-dipole forces

Occurs between neutral polar molecules

Ion-Dipole

Example:

In this example, the oxygen atom in the water molecule has a slightly negative charge and is attracted to the positive sodium ion.

The electrostatic interaction between a charged ion and a neutral molecule that has a dipole

Attraction becomes stronger as either the charge on the ion increases, or as the magnitude of the dipole of the polar molecule increases

Most commonly found in solutions