Dispersion Forces!. By: Lisa, Alyssa, Brandon and Liam. They are the weakest kind of intermolecular attraction and occur between molecules.
By: Lisa, Alyssa, Brandon and Liam
They are the weakest kind of intermolecular attraction and occur between molecules.
They are thought to be caused by the motion of electrons. This is because they are a temporary attractive force that results when the electrons in two adjacent atoms occupy a position that make the atoms form temporary dipoles.What are Dispersion Forces?
The strength of dispersion forces increases as the number of electrons in a molecule increases.
As well as the motion of electrons, the number of electrons in a molecule can affect its dispersion forces.
The more electrons in a molecule, the greater its dispersion forces. A good example of this is the halogen group.
Fluorine and chlorine do not have very many electrons and consequently have relatively weak forces of attraction between their molecules.
The attractions being weak allows the molecules to move around more and that is why fluorine and chlorine are
gasses at STP.Electrons Influencing Dispersion Forces
As you move down the halogen group, bromine has a much greater number of electrons and therefore generates larger dispersion forces between molecules.
This causes the forces of attraction to be greater between the molecules in bromine. They can not move around as much as the molecules in fluorine and chlorine, so bromine is a liquid at STP.Electrons Influencing Dispersion Forces
If we follow the pattern we will see, that iodine and astatine which have the greatest number of electrons in the halogen group, also have the strongest intermolecular forces between their molecules.
The dispersion forces being even greater than with bromine, the molecules can’t move around at all, they can only vibrate.
Bromine and astatine are solids at STP.Electrons Influencing Dispersion Forces
Dispersion Forces are also known as London forces and van der Waals forces.
They were named the London forces in honor of the German physicist; Fritz London who studied these forces!!
Also Known As...
Fritz London (1900-1954)
The name van der Waals forces is a credit to the Dutch chemist and physicist Johannes van der Waals who extensively studied intermolecular forces.
Unlike the name London forces, van der Waals forces refers to the collective grouping of the weakest attractions between molecules.
This includes dispersion forces as well as dipole interactions.
Johannes van Der Waals (1837-1923)
. chemist and physicist Johannes van der Waals who extensively studied intermolecular forces.
We have already learned about dipole interactions. We know that they are a weak intermolecular force and result from the attraction of oppositely charged regions of polar molecules. Now we will learn how dipoles and ions can induce these intermolecular forces.
charged balloon attract the negative charges in your hair.
Ion-induced forces are the result of an ion distorting the electron density of a non-polar molecule.
When an ion comes in close contact with a non-polar molecule the non-polar molecule becomes momentarily polarized, and the two molecules are attracted to each other.Ion-Induced Intermolecular Forces
In covalent bonds, the shared pairs of electrons are constantly vibrating. This happens with all molecules but it is particularly interesting with non-polar molecules.
These bond vibrations actually cause momentary, uneven distributions of charge. So they can actually cause non-polar molecules to become slightly polar for an instant.Temporary-Induced Polar Molecules
When a non-polar molecule becomes polar, even for an instant it is capable of inducing a charge on a nearby molecule (a dipole-induced dipole force).
This as well as ion-induced dipole forces, results in intermolecular forces of attraction, also known as dispersion forces, between molecules!
Polarizability is the relative tendency of a charge distribution to be distorted from its normal shape by an external electric field.
So, in other words the likely-hood that the even distribution of charge will be disrupted within a molecule (by the temporary induction of a dipole or ion) is referred to as the molecule’s polarizability.Polarizability
And In Conclusion... distribution to be distorted from its normal shape by an external electric field.