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Chemistry

Bonding, structures and properties

Intermolecular forces of attraction

Intermolecular forces of attraction are forces between different molecules.

The intermolecular forces of attraction considered here are:

  • Van der Waals' forces
  • permanent dipole-permanent dipole interactions
  • hydrogen bonding

Van der Waals' forces

Van der Waals' forces are forces of attraction which exist between all atoms and molecules. Van der Waals' forces are much weaker than all other types of bonding.

They are only significant in atoms and molecules which have no other types of intermolecular forces of attraction, for example, discrete non-polar molecules and the Group 0 elements.

Van der Waals' forces are a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules.

Van der Waal's forces attract atoms to each other. Atoms have a slight positive charge on the side that is electron deficient, and negative charge on the side where there is electron excess. These positive and negative charges attract one another.

The strength of Van der Waals' forces is related to the size of atoms and molecules.The bigger the atom or molecule the bigger the Van der Waals' force.

Permanent dipole-permanent dipole interactions

The permanent dipole to permanent dipole interaction between two iodine chloride molecules - the positive charge of the chloride ion attracts the negative charge on the iodide ion.

Permanent dipole-permanent dipole interactions are additional electrostatic forces of attraction between polar molecules.

Permanent dipole-permanent dipole interactions are stronger than Van der Waals' forces for molecules of equivalent size.

A molecule can be described as polar if it has a permanent dipole. A permanent dipole is due to a difference in electronegativity between the atoms involved in a covalent bond.

For example:

Permanent dipole in an water molecule showing a negatively charged oxygen atom and positively charged hydrogen atoms
Permanent dipole in tetrachloromethane molecule showing positively charged carbon atom and negatively charged chlorine atoms

The spatial arrangement of polar covalent bonds can result in a molecule being polar.

Non symmetrical spatial arrangement of polar covalent bonds in a water molecule showing negative polarity around the oxygen atom and positive polarity around the hydrogen atoms

If the molecule is symmetrical the molecule will be non-polar.

Symmetrical spatial arrangement of polar covalent bonds in tetrachloromethane resulting in a non-polar molecule. The carbon atom has positive charge and the chlorine atoms have negative charge.

Hydrogen bonding

Hydrogen bonds are permanent dipole-permanent dipole interactions.

Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen are highly polar.

Hydrogen bonds are electrostatic forces of attraction between molecules which contain these highly polar bonds.

Each hydrogen bond is formed between a hydrogen atom in one water molecule and the electronegative oxygen atom of a different water molecule

A hydrogen bond is stronger than other forms of permanent dipole-permanent dipole interactions but weaker than a covalent bond.

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