 In this video, we will take a quick look at the various properties of group 18 elements. How they differ from the other p-block elements in terms of their atomic, physical and chemical properties. So what are the group 18 elements? These are helium, neon, argon, krypton, xenon and radon. So these are the group 18 elements otherwise called noble gases. Now as the name correctly suggests, all the elements here are gaseous in nature. All of these elements are colorless, odorless and tasteless gases. More importantly, they are mono-atomic gases. That is, every molecule of these elements contains only one atom. Unlike oxygen gas where every molecule of the oxygen gas has two atoms of oxygen. Now this is a very important characteristic of group 18 elements that all of these elements are mono-atomic. This further affects the type of chemical interactions or the strength of bonding that can occur in these elements. Secondly, they are noble in nature. That is, these elements are highly chemically unreactive because of their completely filled valence shell electronic configuration. Now as you can see here, there is a highly stable configuration, right? So that means these elements have no tendency to lose or gain electrons because on doing so, this stability will get lost. And as a result, these elements form very few compounds and only under specific conditions and mostly with highly electronegative elements like oxygen or fluorine. Now having completely filled valence shell electronic configuration also affect their atomic properties like ionization in thalpy. For example, these elements exhibit a very high ionization in thalpy. Obviously, because they have a completely filled octet electronic configuration, right NS2-NP6 is their outer electronic configuration and that means they have very little tendency to lose electrons. Therefore, we have to provide a very high amount of energy to disrupt this stable electronic configuration. For the exact same reason, they also have a very large positive electron gain in thalpy. That is, we need to supply a large amount of energy to add an electron to group 18 elements. Now if you compare it or contrast it with the immediate predecessors which is the halogens, you will see that halogens have a very high negative electron gain in thalpy. That means a large amount of energy is released when an electron gets added to the halogen atom. And this we can figure out or we can reason this by simply looking at their electronic configurations. The outer electronic configuration of halogen is NS2-NP5 whereas that of group 18 is NS2-NP6 right? That is an addition of an electron will result in a highly stable state for halogen. But in the case of a noble gas element, the electronic configuration is already very stable. So adding an extra electron will only make this unstable right? For example, if you look at argon, the outer electronic configuration is 3S2-3P6 and if you add an extra electron, what would be the new electronic configuration? It would be 3S2-3P6 and 4S1 right? This is the electronic configuration of potassium which as we know is a highly reactive alkali metal. This is why in the case of group 18 elements to add an electron, energy is not released but rather consumed. Ok so we are done with ionization enthalpy and electron gain enthalpy. Let's now look at atomic radius. Being monoatomic gasses, the non-bonded radii of group 18 elements is very large. That is why we usually compare their atomic radii with van der Waal radii of other elements and not covalent radii. You see, van der Waal radius is measured when there is no bonding between the two atoms. It measures the closest distance between two non-bonded atoms. Covalent radius on the other hand is measured when the two atoms are bonded with each other. Covalent radii is half the inter-nuclear distance between two single but bonded atoms in a molecule. Now noble gasses are monoatomic and are not found in combined state. So it makes more sense to compare their atomic radii with van der Waal radii of other elements rather than covalent radii. Now being monoatomic the only type of interactions possible in these noble gasses is the weak dispersion forces. And because of these weak inter-atomic forces noble gasses in general have very low melting and boiling points. In fact helium has the lowest boiling point of any known substance and has this unique ability to diffuse through materials like glass, rubber, plastic and so on. Obviously as you can imagine this makes it very difficult to work with helium in laboratories. Now even the noble gasses are chemically inert and unreactive we still have a few compounds of these elements especially that of xenon. There are many xenon fluorine compounds like Xcf2, Xcf4, Xcf6 and similarly many xenon oxygen compounds like XeO3, XeO2, F2 and so on. Now the reason why we do not find compounds of helium, neon or argon is because their ionization enthalpies are much higher than that of xenon. The ionization energy for krypton is slightly lower than that of xenon and krypton does form compounds like krf2. Now as radon is highly radioactive and has no stable isotopes a lot of work has not been done on this particular element only rnf2 and a few other complexes are known. Now needless to say that none of these compounds are actually naturally occurring we force xenon to react under specific experimental conditions to form these compounds and it was a scientist named Neil Bartlett who succeeded at making this happen. He observed that the first ionization enthalpy of xenon was almost similar to that of the first ionization enthalpy of oxygen so what he did was that he replaced oxygen in this particular reaction with xenon and on doing so he observed that the gases xenon and ptf6 almost immediately combined at room temperature to form a yellow colored solid. Now this yellow solid was initially thought to be xc plus ptf6 minus exactly similar to the oxygen counterpart simply replaced oxygen with that of xenon but later experiments suggest that it was probably a mixture of platinum complex and several xenon cations with this particular complex being the major product. Now after this particular discovery a number of xenon compounds were prepared in the laboratory especially with highly electronegative elements like oxygen and fluorine.