What are Dispersion Forces?

1. What are Dispersion Forces? • 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. The strength of dispersion forces increases as the number of electrons in a molecule increases.

2. Representing A Dispersion Force • The dotted lines in between these two polar molecules represent an electrostatic attraction (a dispersion force). • Dispersion forces are present between molecules when they are almost touching. They are present between both non-polar and polar molecular compounds and ionic compounds.

3. Electrons Influencing Dispersion Forces GASES • 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.

4. Electrons Influencing Dispersion Forces LIQUIDS • 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.

5. Electrons Influencing Dispersion Forces SOLIDS • 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.

6. Also Known As... • 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!! Fritz London (1900-1954)

7. 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)

8. Induced Intermolecular Forces • When a charge on one object distorts the distribution of charge on a nearby object, an induction of charge occurs and a force of attraction results between the two objects. • A simple example of this is rubbing a balloon on your shirt and causing static in your hair. • Rubbing the balloon against your shirt causes it to loose electrons and become positively charged. When you hold the balloon up to your hair it distorts the even distribution of charge in the molecules in your hair. • The ions in the positively charged balloon attract the negative charges in your hair.

9. Induced Intermolecular Forces In Molecules • In this same way, when an outside charged field is applied to a molecule it becomes an induced dipole or polar molecule, with an uneven distribution of charge. • This happens because the cloud of electrons that surrounds the nucleus of an atom is displaced by the outside field.

10. Ion-Induced Intermolecular Forces • 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.

11. Dipole-Induced Intermolecular Forces • A dipole-induced force is similar to an ion-induced force but in this case it is the actual charged polar molecule that is responsible for inducing the charge on the non-polar molecule. • The induction of charge by dipoles results in what is similar to a chain reaction. The newly induced dipole can induce another non-polar molecule nearby and so on. http://www2.wwnorton.com/college/chemistry/gilbert/tutorials/ch9.htm

12. Temporary-Induced Polar Molecules • 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.

13. 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!

14. Polarizability • 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.

15. Easily Polarized Molecules • If a molecule can be easily polarized it’s because it is large and/ or has a lot of electrons (so more chance of an induction of charge). • If a molecule is easily polarized, then the London dispersion forces between it and a neighboring molecule are normally stronger. • HCl has an uneven electron distribution within the molecule, giving rise to strong dispersion forces between molecules, and easy polarizability.

16. Not As Easily Polarized Molecules • If a molecule is not as easily polarized because it is smaller and/ or there are less electrons (so less chance of a temporary induction of charge) then the London dispersion forces between it and a nearby molecule will tend to be weaker. • The attraction between two H2 molecules is very weak. The dispersion forces are not as strong because there is less change of an uneven distribution of charge occurring. They are not easily polarized molecules.

17. Melting and Boiling Points • Solids are able to melt, and liquid substances are able to boil, when there is enough energy present for particles to overcome their forces of attraction with surrounding particles to break free and move around more. • For substances to melt or boil they have to break the intermolecular forcesbetween their particles. If the particles can break free and move around more, then their state will change. i.e.: solids  liquids and liquids  gases • As you increase the temperature of a substance, you increase the energy in its ions or molecules. So depending on how strong the forces of attraction are between particles, a substance may require more energy (higher temperatures) in order to break the forces

18. Factors That Cause High Melting and Boiling Points • Larger molecules that have more electrons have more vibrations within their molecules. This means that they can easily cause an uneven distribution of charge within their molecules. • This increases the strength of the dispersion forces between larger molecules, making their forces stronger, and a lot more difficult to break. • Their attractions are stronger, so they require a more energy to break them apart which means higher melting and boiling points.

19. Factors That Cause Low Melting and Boiling Points • The shape of molecules can also affect their boiling and melting point. • Molecules with a spherical shape have a smaller surface area than a straight chain molecule even if they have the same number of electrons. • A smaller surface area means there is less chance of causing an uneven distribution of charge within the molecule. This also means less opportunities to induce a charge on a nearby molecule. • Therefore, substances with more spherical shaped molecules will have weaker dispersion forces. • Weaker dispersion forces mean that there is less energy (lower temperatures) required to break the forces holding molecules together.

20. And In Conclusion... • Dispersion forces are intermolecular attractions that are responsible for the bonds between molecules and ions. • They can be induced by surrounding electric fields and the electrons in an atom can help you predict how strong dispersion forces will be. • Without intermolecular forces, molecular compounds would only exist as gases because there would be nothing to hold the molecules together. • .