in this video we're going to talk about london dispersion forces these are forces that are found in every type of atom every type of molecule however they are the predominant force found in nonpolar molecules so let's say if we have an atom with a nucleus with a positive charge and there are electrons around the atom now this atom is nonpolar the electrons are evenly distributed among this atom let's say it's in the gas phase and the net charge is positive six so this atom is a nonpolar atom due to the even distribution of electrons now as these electrons are constantly moving about in the atom there's going to be a moment where there's going to be more electrons on one side as opposed to the other side so due to the random motion of the electrons about the nucleus at some point most of the electrons might be on the left side and some will be on the right side and so now we have an uneven distribution of electrons so this atom is temporarily polarized it's polarized in the sense that one side of the atom the side that is electron deficient has a partial positive charge and side that has more electrons has a partial negative charge so right now this atom is a dipole it's a temporary dipole it doesn't last very long it lasts for a very short time and this dipole can cause another neutral atom to turn into a dipole so it can induce another dipole in a similar molecule so let's put another atom next to it in the gas phase so this atom is neutral the electrons are evenly distributed so it's nonpolar like this one now the electrons will be attracted to the partial positive charge of the right side of that atom so therefore the electrons will feel a force of attraction and so this electron cloud will become distorted and so we're going to have this situation the first atom is going to be the same it still has its temporary dipole and now the second atom is polarized as well it has an induced dipole so now this side of the atom which has a partial positive charge is attracted to the left side of the second atom with its partial negative charge and so that force of attraction that connects those two atoms this is the london dispersion force it's a temporary dipole-induced dipoint of action so this dipole caused this atom to be induced into a dipole so this is an induced dipole it was created by the other atom and so this interaction is temporary short-lived it doesn't last very long now london dispersion forces is dependent on the number of electrons that's found in an atom so for example iodine which has about 53 electrons is more polarizable than fluorine which has about nine electrons and so iodine is a bigger atom than fluorine and typically larger molecules with more electrons are more polarizable than smaller molecules the term polarizability means that the electron cloud can easily become distorted so the more electrons you have as they move randomly about the atom the chances of an induced dipole forming or an instantaneous dipole will increase because the more electrons you have there's going to be an increased probability that one side is going to have more electrons than the other side so the chances of distorting the electron cloud goes up so london dispersion forces increases with increased polarizability and the polarizability of an atom is dependent on the number of electrons found in the atom so what you need to know is this nonpolar molecules with more electrons have more london dispersion forces than non-polar molecules with less electrons consider the noble gases helium neon argon and krypton so which of these molecules would you expect to have the greatest london dispersion forces helium contains two electrons neon has 10 electrons argon has 18 electrons and krypton has 36. so therefore we should expect that krypton should have the highest london dispersion forces in fact the boiling points of these gases are as follows the boiling point for helium is negative 269 celsius for neon it's negative 249 celsius for argon it's negative 186 and for krypton negative 153. so what general trend do you notice notice that the boiling point increases with increasing ldf so the more dispersion forces that a molecule has the greater the boiling point will be so as the number of electrons within an atom or within a molecule increases the boiling point of that substance will increase due to the increased amount of intermolecular forces specifically london dispersion forces now keep in mind the london dispersion force is a type of intermolecular force other intermolecular forces include dipole interactions and hydrogen bonds now consider these four molecules fluorine chlorine bromine and iodine which of these would you expect to have the highest boiling point well first let's find out which one has the most number of electrons now every fluorine atom has 9 electrons so if we multiply that by 2 we would get 18 electrons in the fluorine molecule now chlorine atom has 17 electrons so this would be 34 in total bromine has 35 so that's going to be 70 in total and iodine has 53 times 2 so that's going to be 106 in total so we know that the molecule with the most number of electrons is the one that's going to have the highest amount of london dispersion forces so therefore we should expect that iodine will have the highest boiling point in fact iodine is a purple solid bromine is a red liquid chlorine and fluorine are gases solids usually have a very high boiling point and gases tend to have a very low boiling point the boiling point of fluorine is negative 188 degrees celsius it's very low the boiling point of chlorine is negative 34. the boiling point of bromine is 59 which makes sense it's a liquid and the boiling point of iodine is 184 degrees celsius so as you can see iodine has the highest boiling point because it has the most amount of london dispersion forces it has the greatest amount of electrons so which means the iodine is highly polarizable now let's consider one more example so i'm going to draw two molecules and i want you to determine which of these two molecules has the higher boiling point so on the left this substance is known as pentane on the right it's neopentine so which molecule would you expect to have the higher boiling point now both molecules have the same molecular formula they both have five carbons and 12 hydrogens so therefore they have the same number of electrons however they are different in one aspect this is a straight chained alkane and here we have a branched alkane it turns out that pentane has a higher boiling point than neopentane the boiling point of pentane is 36 celsius and the boiling point of neopentane is 9.5 celsius pentane looks like this it's a straight chain alkane neopentane is more circular so notice that pentane has more surface area than a neopentane and that's why it has a higher boiling point so what you need to know is this whenever you increase the surface area of a substance the amount of london dispersion forces between those molecules will increase and as a result the boiling point will increase as well so molecules that are straight change tend to have a higher boiling point than molecules that are branched you