in this lesson we're going to talk about Vander Waal forces and London dispersion forces now these forces they exist in all molecules but there are the predominant forces in nonpolar molecules so let me describe what they are with a visual illustration so let's say we have two atoms with a positive charge at the centre and we know that in the electron cloud the atoms are surrounded by a negative charge the nucleus at the centre has a positive charge now dependent on the number of protons it can be a positive eight charge positive twelve charge we're not going to focus on that particular number so when the electrons are evenly distributed in an atom you're gonna have a non-polar atom now because the electrons are constantly moving there's gonna be times where one side of the atom will have more electrons than the other side in this case the electron cloud is now distorted so we have an excess of negative charge on the left and a deficiency of negative charge on the right side so basically we can say that this particular picture that looks like this so there's a net negative charge on the left and because the right side is electron deficient it's more positively charged down now granted there's still negative charge on the right side but this positive charge means that is less negative than the left side so now this atom is now polarized it's now a dipole now this dipole doesn't last long so it may last for a very very very short time so it's a temporary dipole what happens if we take this dipole and place it next to an polar atom where the electron cloud is evenly distributed the electrons will full of force that will accelerate them towards the positive side of that atom and so this electron cloud becomes distorted and so this atom becomes polarized and so what happens is now you have two polarized atoms now you need to be familiar with the term called polarizability and it basically describes the probability of an atom's electron cloud being distorted due to the random motion of electrons so let's compare fluorine and iodine fluorine has nine electrons iodine has 53 so because iodine has a lot more electrons than fluorine it's more polarizable it can create a temporary dipole more likely than a fluorine atom so a temporary dipole will be more likely to form in an iodine atom than a fluorine atom now notice that the positive side of this atom will be attracted to the negative side of the other one so these two they will feel a force of attraction that accelerates them and so these forces that attracts these atoms together they're known collectively as the Vander Waal forces also known as the London dispersion forces lbf now this first example it was a temporary dipole a dipole is basically a polarized object or polarized molecule where one side is negative and the other side is positive now the reason why it's a temporary dipole is because it can go back to its original state the electrons on the left side can diffuse towards the right side because this negative charge is attracted to the positive charge and so it could be turned into it's nonpolar state where the electron cloud is no longer distorted the electrons are distributed evenly so keep that in mind this temporary dipole doesn't really last long it's dependent on the size of the atoms now the dipole on the right is an induced dipole and the reason why it's called an induced dipole is because it was induced or created by this first dipole when this dipole became when the electron cloud of that dipole became distorted it caused this the electric lock for this atom to become distorted thus creating an induced dipole so make sure you understand the word induced dipole adjust the dipole that was created by something else so it doesn't last long it can revert back to its nonpolar state and so these forces that hold or that attract these dipole induced dipole molecules together and those are the Vander Waal forces or the London dispersion forces they're very weak and they're found in every molecule however in nonpolar molecules and they are the dominant forces in nonpolar molecules so these Vander Waal forces explain why some nonpolar molecules have higher boiling points than other nonpolar molecules let's consider the hill urchins for example fluorine chlorine bromine and iodine fluorine is a gas chlorine is a gas bromine is a red liquid and iodine is basically a purple solid at room temperature of course now the boiling point of fluorine is negative 100 88 degrees Celsius the boiling point of chlorine it's negative 34 degrees Celsius and for bromine it's 59 degrees Celsius and for iodine it's even higher as 114 degrees Celsius now if we write down a molecular weight of these molecules let me get my book out the molar mass or molecular weight for fluorine is 38 fluorine has 9 electrons as we said the molar mass for chlorine is approximately 70 point 9 and chlorine as an atom has 17 electrons bro means molar mass this almost is like 150 9 point 8 and it has 35 electrons per and for iodine we said had 53 electrons and it's molar mass is about 250 3.8 so as we travel down the periodic table in group 7a that is the group where all the halogens are notice that the number of electrons increases and so the molecules are becoming more polarizable due to the increased number of electrons and so the molecular weight and the number of electrons they're proportional so as a molecules molecular weight increases the number of electrons will increase with it too now with molecules that have a high number of electrons or a higher molecular weight those electrons will I mean those molecules will have more London dispersion forces or more LDF forces or you could say van der Waal forces they're the same and so as that goes up you could see the trend with the boiling point the boiling point increases so iodine has the highest so make sure you understand this relationship so molecules with more electrons or higher molar mass values they will have more Vander Waal forces and thus they will have a higher boiling point so boiling point is directly related to the amount of LDF forces that a molecule has so here's a question for you consider the molecules methane ethane propane and butane so which of the following four molecules will have the highest boiling point so looking at the chemical formula this is C 1 H for ethane has two carbons six hydrogen's propane has three carbons eight hydrogen's and butane has four carbons and 10 hydrogen's so if we analyze the molar mass of each of these molecules carbon is about 12 hydrogen is 1 so methane has a molar mass of 16 methane is going to be 2 times 12 that's 24 + 6 so approximately 33 times 12 is 36 plus 844 4 times 12 is 48 plus 10 so 58 so the molecule with the highest molar mass is gonna have the highest boiling point so it's gonna be Beauty methane has the lowest boiling point because it has the lowest molar mass all of these molecules are nonpolar anytime you have a hydrocarbon that contains only carbon and hydrogen bonds it's a non-polar molecule therefore the predominant intermolecular force acting on it will be London dispersion forces or van der Waal forces