SCH 3U1 ATTRACTIVE FORCES
There are different types of forces that exist between molecules and particles. The strength of these particles determines the state of the substance. For example, if a substance is solid that means the particles are locked beside each other tightly. This is a strong force. In the case of gases the forces are weak. The particles themselves move about freely and when they collide they rebound off each other.
Electrostatic Force: This force is referred to as an ion-ion interaction or an ionic bond. It is the result of a positive and a negative charge wanting to pull closer to each other. These particles usually form ionic lattices.
Dipole-Dipole Force: This force occurs when a polar molecule interacts with a polar molecule. A polar molecule means that the electrons are shifted toward the more electronegative element, eg. HCl. In this molecule the electrons are pulled toward the chlorine, making the chlorine (slightly) negative and the hydrogen positive. The hydrogen chloride molecule is said to have a dipole.
 +  -  +  -  +  -  +  -  +  -  +  -
H-Cl --- H-Cl --- H-Cl --- H-Cl --- H-Cl --- H-Cl
The three dashes indicate the dipole force that exists.
(Analogy: a layer of butter between two oreo cookies.)
Dispersion Forces: In an atom or a non-polar molecule the electron cloud is symmetrical. However, the electrons have a random motion within the electron cloud. In any one instant there may be more electrons on one side of the cloud. This will give it a (brief and slight) charge. This can induce a charge in a neighbouring particle. These interactions that form keep the liquid together and have to be broken before it becomes a gas.