Van der Waals Forces theory

Van der Waals forces, although they don’t form a chemical bond theirselves, they are responsible for the attraction between neutral and saturated molecules, which result in the condensed states (liquid and solid) of these substances.

The Van der Waals force are very weak when the molecules are electrically symmetrical, for example with (non-polar or nonpolar) as it happens por example, in the monatomic molecules, noble gases, and in homonuclear diatomic molecules (H₂, N₂, F₂, etc) and in symmetric polyatomic molecules (CO₂, CS₂, CH₄, etc).

Instant dipoles and induced dipoles:

It is probably that in particular moment, only by chance, electrons are concentrated in a region of an atom or molecule. This electron displacement makes a normally non-polar species becoming into momentarily polar. It has formed an instantaneous dipole. That is said, the molecule has a dipole moment instant. After this, the electrons of an atom or molecule can travel to neighboring also producing a dipole. This is a process of induction and the new dipole formed is called “induced dipole“.

These two processes, considered jointly, lead to an intermolecular attractive force. This force can be called as instant attraction force dipole/dipole induced but the most commonly used names are dispersion force and London force.

The term “polarizability” is the term used to describe the tendency to produce a charge separation in one molecule. The higher this trend, it is said that the molecule is more polarizable. The polarizability increases with the number of electrons and the number of electrons increases with the molecular mass. Also, in large molecules, some electrons are further from the core and therefore they are less firmly attached to them. These electrons move more easily and the polarizability of the molecule increases. Due to dispersion forces are stronger when the polarizability increases, the molecules attract each other more strongly, with the result of increasing the melting points and boiling covalent substances by increasing the molecular mass. For example, helium, with a molecular mass (atomic) 4 u, it has a boiling point of 4 K, while radon (atomic mass 222 u) has a boiling point of 211 K.

The intensity dispersion forces also depends on the molecular form. Electrons in the elongated molecules can move more easily than the small, compact and symmetric molecules; elongated molecules are more polarizable. Two substances, with the same number and kind of atoms but with different molecular forms, isomers, they may have different properties.

Dipole-dipole interactions:
In a polar substance, the molecules have permanent dipole moments. The result is that the molecules try to align themselves with the positive end of a dipole directed toward the negative ends of neighboring dipoles. This partial ordering of molecules can make that a substance remains as solid or liquid at higher temperatures than what it is expected.

Regarding the importance of the Van der Waals forces, we have to consider that:

• Dispersion forces (London) exist in all types of molecules. They involve the movement of all electrons in molecules and increase with the molecular mass increasing. They also depend on the shape of the molecule.
• The forces associated with permanent dipoles involving the movement of electron pairs links instead of global shifts in molecules. Only they are found in substances resulting dipole moments, polar molecules. Its effect is added to the dispersion forces, also present.
• When substances of similar molecular masses are compared, the forces between dipoles can produce significant differences in properties such as melting point, boiling point and enthalpy of vaporization.
• When substances with very different molecular masses are compared, dispersion forces are usually more important than the dipole strengths.

In the case of HCl and F₂ their masses are comparable, but due to the HCl is polar, it has a value of enthalpy of vaporization (ΔH_vap) higher and a boiling point higher than the F₂ has. Within the HCl, HBr and HI series, the molar mass increases rapidly and the enthalpy of vaporization and boiling point increase in the order HCl <HBr <HI. The more polar character of HCl and HBr in relation to HI is not sufficient to reverse the trends produced by the increase of molecular masses; dispersion forces are the predominant intermolecular forces.

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