Intermolecular forces occur between molecules, whereas a bond is a force within a molecule. Bonds are considerably stronger than intermolecular forces. Intermolecular forces originate from interactions between charges, partial charges, and temporary charges on different molecules or atoms. We have four types of intermolecular forces. Dispersion forces, which are also known as London dispersion forces or van der Waals forces, are generally the weakest. Then we have dipole-dipole forces which occur between two molecules with permanent dipoles. And hydrogen bonding, which is the strongest intermolecular force in a pure substance. In mixtures, such as ionic solids dissolved into water, we can have ion-dipole forces, which are even stronger. Dispersion forces are the result of an instantaneous dipole interacting with an induced dipole. Dispersion forces are present between all molecules and atoms but are the only intermolecular force between nonpolar molecules. For a simple thought experiment, let's consider solid helium. In our solid, our atoms of helium are electrically neutral and there are no permanent dipoles in the helium atom. A single helium atom has two electrons and those electrons have to be somewhere. In one instant, those electrons may be on opposite sides of the nucleus. But let's say in another instant (depicted in frame three) the electrons are both closer to one side of the atom than the other. In that instant, we set up what's called an instantaneous dipole. This dipole may only last for a tiny fraction of a second: an instant. But at that instant, we have a separation of the center of positive and negative charge in our atom and we set up in instantaneous dipole. Now let's say that helium atom with the instantaneous dipole is in our solid, so it's next to other helium atoms. That instantaneous dipole can induce a dipole in a neighboring atom. For example, the partial positive in the instantaneous dipole can attract the electrons of the neighboring atom to the left side of that atom, thus setting up an induced dipole. The induced dipole may only last for a tiny fraction of a second, but it causes an attraction between the two helium atoms. In order to transition from a liquid to a gas, we need to overcome the intermolecular forces that keep the liquid particles close to one another. So in general, as intermolecular forces increase, boiling points increase. The boiling point of helium is 4.2 Kelvin. This means even an incredibly small amount of thermal energy can overcome all the intermolecular forces in helium. So dispersion forces are very weak in noble gases. But in general, dispersion forces increase with increasing molar mass. As atoms or molecules get larger, they have more and more electrons and those electrons are held farther from the nucleus. This makes larger atoms more polarizable: that is, it's easier to set up these instantaneous and induced dipoles. This is why our boiling points increase for noble gases going down a column. Our noble gases are getting larger and more polarizable, increasing their intermolecular forces. Dispersion forces also increase with increasing surface area because there's more opportunity to set up instantaneous and induced dipoles. We can see this by comparing the boiling points of n-pentane and neopentane. The boiling point of n-pentane is 36.1°C, whereas the boiling point of neopentane is 9.5°C. Both are composed of the exact same atoms, so they have the same molar mass. The only difference is that the carbon atoms in n-pentane are connected in a line, and the carbon atoms and neopentane are connected to essential carbon atom. The additional surface area in n-pentane increases the dispersion forces and thus it increases the boiling point. For similar molecules composed of the same elements, larger molecules have higher boiling points. We can see this trend by comparing hydrocarbons of increasing lengths. As we have longer chains of carbon and hydrogen atoms, the boiling points increase. Our next intermolecular force—dipole-dipole forces—exist between molecules that are polar, which have a permanent dipole. An example is acetone. In acetone, the carbon-oxygen double bond is polar. The electrons spend more time near the oxygen atom than the carbon atom because oxygen is more electronegative. So oxygen has a partial negative and carbon a partial positive. The partial negative of one acetone molecule will be attracted to the partial positive of another acetone molecule. This is the dipole-dipole force. Dipole-dipole forces are generally stronger than dispersion forces. Formaldehyde and ethane have similar molar masses, but formaldehyde is a polar molecule, so there are dipole-dipole forces. These additional intermolecular forces contribute to the higher boiling points and melting points of formaldehyde compared to ethane. Dipole-dipole forces increase with increasing strength of the dipole moment. For this series of compounds with similar molar masses, we can see the increasing boiling points with increasing dipole moments. The polarity of molecules is also important in miscibility, which is the ability of two liquids to be mixed without separating. Polar molecules are generally miscible with other polar molecules, and nonpolar molecules are miscible with other nonpolar molecules. Hydrogen bonding occurs between two molecules that have a hydrogen directly bonded to either a nitrogen, oxygen, or fluorine atom. Hydrogen bonds are not actually bonds: they are intermolecular forces. You can think of hydrogen bonding as particularly strong dipole-dipole forces because of the large electronegativity differences between hydrogen and nitrogen, oxygen, or fluorine and the relatively small size of these atoms. Hydrogen bonds are the strongest intermolecular force present in a pure substance. We can see the effect of hydrogen bonding when we compare the boiling points and melting points for ethanol and dimethyl ether. These molecules have the same atoms but in ethanol, the atoms are arranged so that there is a hydrogen atom directly bonded to an oxygen atom. In dimethyl ether, the oxygen atom is bonded to 2 carbon atoms instead, so we have hydrogen bonding in ethanol but not in dimethyl ether. The boiling points and melting points of ethanol are significantly higher because of the stronger intermolecular forces. We can also see the effect of hydrogen bonding in water. If water did not have hydrogen bonding, we might expect it to have a boiling point somewhere around -80°C based on its molar mass in comparison with the other compounds. But the presence of hydrogen bonding increases the intermolecular forces by so much that the boiling point increases to 100°C. Ion-dipole forces occur between polar molecules like H2O and ions. These forces are very strong and their combined effect can even overcome the electrostatic forces keeping ions in a crystal lattice for an ionic compound. This is how ionic compounds are dissolved in polar solvents. So in summary, dispersion forces occur in all molecules and atoms and are due to instantaneous and induced dipoles. They're generally the weakest intermolecular force. Dipole-dipole forces occur between polar molecules, and hydrogen bonding is the strongest intermolecular force for a pure substance. Hydrogen bonding only occurs when we have a hydrogen directly bonded to a nitrogen, oxygen, or fluorine atom. Ion-dipole forces occur in mixtures of ionic compounds and polar compounds. Let's practice identifying intermolecular forces. We'll start with the Lewis structure for ammonia, NH3. We have a hydrogen atom directly bonded to a nitrogen atom, so hydrogen bonding is present. Ammonia is a polar molecule, so there are also dipole-dipole forces between two ammonia molecules. And we have dispersion forces between all atoms and molecules. Next, let's draw the Lewis structure for xenon tetrafluoride. Our molecular geometry will be square planar, so xenon tetrafluoride is a nonpolar molecule. The only intermolecular forces present between nonpolar molecules are dispersion forces. For CH2F2, we'll start with our Lewis structure. Since our hydrogen atoms are not directly bonded to the fluorine atoms, we do not have hydrogen bonding. However, since it is a polar molecule, we do have dipole-dipole forces. And, of course, we also have dispersion forces.