# Intermolecular Forces **Intermolecular forces** are attractions between different molecules which determine physical properties such as: - Boiling Points - These can refer to the attractive forces between the particles of a substance, regardless of whether they are molecules, atoms, or ions. ## Impact on Phases The phase in which a substance exists depends on the relative extents of its **intermolecular forces (IMFs)** and the **kinetic energies** (KE) of its molecules. - **IMFs** serve to **hold particles close** together - **KEs** provide the energy required to overcome the attractive forces and thus **increase the distance** between particles The differences in the properties of a solid, liquid, or gas reflect the strengths of the attractive forces between the atoms, molecules, or ions that make up each phase. ![[Pasted image 20220604173328.png|300]] > This image illustrates how changes in physical state may be induced by changing the temperature, hence, changing the average kinetic energy, of a given substance. ## Types of Intermolecular Forces **Polar molecules** have two opposite ends with opposite electrical charges. A **dipole** simply refers to the separation of charges within a molecule; it is closely associated with the polarity of a molecule but not the exact same. There are four different types of intermolecular forces: - **Van der Waals** (London Dispersion) - **Dipole-Dipole** - **Hydrogen Bonding** - **Ion-Dipole** ### Van der Waals (London Dispersion Forces/LDFs) London dispersion forces are attractive forces that arise as a result of **temporary dipoles induced** in atoms or molecules > [!important] **All molecules and atoms have london dispersion forces!** ![[Pasted image 20220603102925.png|300]] A neutral molecule will normally have an even distribution of its electron density. However, when one molecule comes into contact with another, the electron cloud will become **temporarily distorted**. This means that as the two molecules get closer together, one side of the electron cloud will become **negatively charged**, while the other side of the same molecule will become **positively charged**. Due to these new temporary dipoles formed in both molecules, the attraction between them tends to increase. ### Dipole-dipole Attractive forces between polar molecules. These are found in molecules that contain a permanent dipole. > These are the **second strongest** type of intermolecular force. ![[Pasted image 20220603103228.png]] ### Hydrogen Bonding Attractive forces between polar molecules that contain a hydrogen atom that is directly bonded to either an oxygen, nitrogen, or fluorine. ![[Pasted image 20220603103540.png]] This is the **strongest of the dipole-dipole interactions** that exists. ### Ion-dipole Attractive forces between an **ion** and a **polar molecule**. > These are the **strongest of all** the intermolecular forces presented here. ![[Pasted image 20220603103913.png|300]] > To the right, we see a sodium cation (positive) that is surrounded by water molecules. You can see by the arrangement, that the ## Other Factors to Consider Larger atoms and molecules exhibit stronger dispersion forces than smaller atoms/molecules. The measure of how easy or difficult it is for another electrostatic charge (for example, a nearby ion or polar molecule) to distort a molecule's charge distribution (its electron cloud) is known as polarizability. > [!info] Molecule Size vs IMF Strength > The **LARGER** the atom or molecule, the **EASIER** it is to polarize (as there is more volume for opposite charges to disperse), therefore the **STRONGER** the intermolecular force. ## The Hydrophobic Effect When nonpolar molecules are present within a polar solvent, they will naturally aggregate accumulation of nonpolar molecules in a polar solvent to maximize the total entropy of the system