DIPOLE INDUCED DIPOLE FORCES

Remember about the force between non-polar molecules, for example argon. The element of argon consists of atom, we call as a monoatomic molecule. In argon atom, there is also an intermolecular force, but this force is really very weak because this atom has no dipole moment. The temporary dipoles are only occurred very rare compare with diatomic molecules, as happened in iodine, I2 molecules.

Now, what would happen if we mixed HCl with argon? The electrons on an argon atom are distributed homogeneously around the nucleus of the atom. When an argon atom comes close to a polar HCl molecule, the electrons can shift to one side of the nucleus to produce a very small dipole moment that occurredin a fraction of time. This is very weak temporary dipoles.
According to this evidence, by distorting the distribution of electrons around the argon atom, the polar HCl molecule induces a small dipole moment on this atom, which creates a weak dipole-induced dipole force of attraction between the HCl molecule and the Ar atom.

DIPOLE FORCES

Now lets discuss about dipole forces. The example of this force is HCl as a polar molecule. Polar molecules are sometimes described as "permanent dipoles" or "dipoles". Each molecule has two "poles" permanently. One end (pole) of the molecule has a partial positive charge while the other end has a partial negative charge. The molecules will orientate themselves so that the opposite charges attract effectively.
In the example on the left, hydrogen chloride is a polar molecule with the partial positive charge on the hydrogen and the partial negative charge on the chlorine. A network of partial + and - charges attract molecules one of another.

IONIC FORCES

The physical properties of melting point and boiling point, for example, are related to the strength of attractive forces between molecules. These attractive forces are called Intermolecular Forces.
In an ionic bonding, there is an ionic force. This force is commonly called electrostatic force (Coulomb).
Each Na+ ion and Cl- ion attracts one of another, so that they forms a Crystal.
Although there is a repulsion between inner electrons of Na+ and outer electrons of Cl-, the ionic force is much stronger than the repulsion.
The ionic force is much stronger than the molecular force, so boiling point and melting point of ionic compounds are higher than covalent compounds. The last force will be discussed later.

INDUCED DIPOLE FORCES

Now lets think about intermolecular forces. There are three different forces, force between non-polar molecules, non-polar and polar molecules, and polar molecules.
Forces between non-polar molecules are the weakest of all intermolecular forces.
In the non-polar molecules there are 'Temporary dipoles" which are formed by the shifting of electron clouds within molecules. These temporary dipoles attract or repel the electron clouds of non-polar molecules.
The temporary dipoles exist for a fraction of a second, so a force of attraction exist for that fraction of time. The strength of induced dipole forces depends on how easily electron clouds can be distorted. Atoms or molecules which have many electrons are more easily distorted, because these electrons are far from their nucleus.
Okay, please explain about intermolecular forces in iodine molecules as shown in the picture. Give the reason why in the iodine molecule, atom that has the negative temporary pole is bigger than atom with positive temporary pole,

HYDROGEN BONDING

Water consists of polar molecules and these molecules attract each other as shown in the picture. This attraction is named hydrogen bond as the strongest intermolecular force.
Remember the intermolecular forces between HCl molecules. Why does the force between H2O molecules is stronger than HCl molecules?
You have to think the electronegativity of O and Cl. The electronegativity of oxygen atom is greater than chlorine atom, so that H2O molecules are more polar than HCl molecules.
How about fluor and nitrogen atoms? Electronegativity of Fluor atom is greater than oxygen atom and oxygen atom is greater than nitrogen atom. EN F> O > N. Are these elements have a hydrogen bonding in their compounds with hydrogen atoms? Certainly yes, they are. Because all these elements have the greatest electronegativity. Please think about HF and NH3, and alcohol molecules.