Intermolecular forces are the forces of attractions that exist between molecules in a compound. These cause the compound to exist in a certain state of matter: solid, liquid, or gas; and affect the melting and boiling points of compounds as well as the solubilities of one substance in another.

Solid: A state of matter in which the matter is not compressible nor does it flow.

Liquid: A state of matter in which the matter is not compressible but can flow.

Gas: A state of matter in which the matter is compressible and can flow.

The melting point of a compound is the temperature at which a compound turns from a solid to a liquid or a liquid to a solid.

The boiling point of a compound is the temperature at which a compound turns from a liquid to a gas or a gas to a liquid. This temperature is a true measure of the forces of attractions between molecules as molecules separate from one another when they turn from a liquid to a gas.

The stronger the attractions between particles (molecules or ions), the more difficult it will be to separate the particles. When substances melt, the particles are still close to one another but the forces of attraction that held the particles rigidly together in the solid state have been sufficiently overcome to allow the particles to move. When substances boil, the particles are completely separated from one another and the attractions between molecules are completely overcome. The energy required to cause substances to melt and to boil, and thus disrupt the forces of attraction, comes from the environment surrounding the material. If you place a piece of ice in your hand, the ice will melt more quickly than if it is placed on a cold counter top. The energy required to melt the ice comes from your hand, your hand gets colder and the ice gets warmer.

Look at the table of melting points and boiling points for the halogens, shown below.

Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights
Substance	FW (g/mol)	mp (°C)	bp (°C)
F2	38	-220	-188
Cl2	71	-100.98	-34.6
Br2	160	-7.2	58.78
I2	254	113.5	184.35

As the size of the halogens increases, the melting and boiling points increase. The energy required to move and separate the molecules from one another increases as the size of the molecules increases. If it takes more energy to separate the molecules, the attractions between molecules must be greater. The types of intermolecular forces responsible for the increase in melting points and boiling points of these non-polar covalent compounds are called London forces or dispersion forces.

Now look at the table below.

Melting Points and Boiling Points of Substances with Similar Formula Weights
Substance	FW (g/mol)	mp (°C)	bp (°C)
F2	38	-220	-188
NO	30	-164	-152
CH3OH	32	-94	65
Ca	40	893	1484
NaF	42	993	1695

All the substances in this table have similar formula weights thus they have similar London forces. If the only attractions between substances have to do with size, then they should have similar melting points and boiling points. They do not. Let us look more closely at the nature of the substance to see if we can relate the structure of the material with its properties.

Fluorine and Nitrogen Monoxide

Fluorine and nitrogen monoxide are similar in size and thus have similar London forces.  Fluorine is a non-polar covalent molecule while nitrogen monoxide is a polar covalent molecule - it has a positive and a negative end, like a magnet. Since nitrogen monoxide has the higher melting point and boiling point, it must have the stronger intermolecular forces. Given the same size, polar covalent molecules must have stronger forces of attraction than non-polar covalent molecules. These forces of attractions are called dipole-dipole forces.

Nitrogen Monoxide and Methanol

Nitrogen monoxide and methanol are similar in size and thus have similar London forces. Nitrogen monoxide and methanol are polar covalent molecules and thus have dipole-dipole forces. Since methanol has the higher melting point and boiling point, it must have the stronger intermolecular forces. The difference in these molecules is the presence of a certain extremely polar bond present in methanol that is not present in nitrogen monoxide. This is the oxygen - hydrogen bond.

Oxygen is more electronegative than hydrogen and pulls the electron density in the oxygen - hydrogen bond towards it. This leaves very little electron density around the hydrogen since hydrogen has no core electrons. The part of hydrogen directed away from the oxygen - hydrogen bond has very little electron density shielding the nucleus. Thus that part of the hydrogen nucleus which is exposed can interact with the non-bonding electrons on another methanol molecule. This interaction of a non-bonding pair with a hydrogen attached to an electronegative element such as oxygen is called a hydrogen bond.

Calcium and Sodium Fluoride

A large jump in melting points and boiling points is observed when we turn from covalent compounds to metals and ionic compounds. Both metals and ionic compounds involve the interaction of particles with full charges.

• Metals. Metal ions interact with the sea of electrons that surround them. This attraction must be very strong as the melting point and boiling point of calcium is much higher than the covalent compounds which share a similar formula weight.
  
  
• Ionic Compounds. Substances which bear full charges, anions and cations, are attracted very strongly as evidenced by the melting point and boiling point of sodium fluoride.

Intermolecular Forces

• Ionic Compounds and Metals
  
  
  • Electrostatic forces - these forces occur between charged species and are responsible for the extremely high melting and boiling points of ionic compounds and metals.
  
  
• Covalent Compounds
  
  
  • London forces - all molecules have the capability to form London forces. These are solely dependent on the surface area and the polarizability of the surface of the molecule. These are the only types of forces that non-polar covalent molecules can form. They result from the movement of the electrons in the molecule which generates temporary positive and negative regions in the molecule.
    
    

    

  • Dipole-dipole forces - only polar covalent molecules have the ability to form dipole-dipole attractions between molecules. Polar covalent molecules act as little magnets, they have positive ends and negative ends which attract each other.
    
    

    

  • Hydrogen bonding - these occur between polar covalent molecules that possess a hydrogen bonded to an extremely electronegative element, specifically - N, O, and F.

    

1. Lewis Structures
2. Formal Charge
3. Resonance Structures
4. Valence Shell Electron Pair Repulsion Theory
5. Bond and Molecular Polarity
6. Intermolecular Forces
7. VSEPR Tutorial
8. VSEPR Quiz

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