 As we have discussed many times by now, the first member of every P-blog enjoys a kind of special attention and our oxygen is no different. Compared to the rest of the group 16 elements, oxygen differs in certain behaviours. In this video, we will look at a few things. We will look at the multiple oxidation states exhibited by oxygen, secondly we will look at the physical properties of oxygen and compare them with our obsulfur and thirdly we will see how the absence of D orbitals affects the chemistry of oxygen and lastly and more importantly we look at a unique property that oxygen has due to its small size and very high electronegativity. We know that oxygen is the second most electronegative element and because of that it tends to show mostly the negative oxidation states which in most cases is minus 2. For example in sodium oxide, potassium oxides, H2O, water, carbon dioxide in all of these you can see that the oxidation state exhibited by oxygen is minus 2. Now oxygen also exhibits minus 1 oxidation state in peroxides like sodium peroxide and potassium peroxide as you can see here. You must be surprised to see that oxygen also shows positive oxidation states like plus 1 and plus 2. So that means it must be combined with something or an element that is more electronegative than oxygen itself right and only then it would be positive, only then it would be willing to give away its electron to only a more electronegative element and which element can be more electronegative than oxygen? Yes, fluorine right? So when oxygen combines with fluorine it attains a positive oxidation state as you can see in this case. In O2F2 it has a plus 1 oxidation state whereas in OF2 it has a plus 2 oxidation state. Both of which you can see that it has combined with a more electronegative fluorine atom. So these are the only instances where oxygen shows a positive oxidation state. Now if you look at the physical properties like melting and boiling point you will notice that there is a huge jump from oxygen to sulfur in both the cases. In general we know that the melting point and boiling point increase as we go down the group with increase in the atomic size. That means we need to provide more energy to break the molecule or the attractive forces that hold them. Now when you compare oxygen and sulfur how can you substantiate this drastic increase in their melting and boiling points? Well that's where atomicity comes into play. Atomicity is nothing but the number of atoms that are present in the molecule of an element. For example, oxygen exists as a diatomic gas that contains two oxygen atoms in a single oxygen molecule. But sulfur is a polyatomic molecule which has eight sulfur atoms linked together. So that means to break the sulfur molecule we need to provide a lot more energy than what is required to break up the oxygen molecule. Exactly. Using atomicity we can explain this large difference in the melting and boiling point between oxygen and sulfur. Another factor that affects the chemistry of oxygen is the absence of empty diorbitals. And just like in the case of boron and carbon and nitrogen this unavailability of diorbitals restricts the covalency of oxygen to 4. You see the electronic configuration of oxygen is 1s2, 2s2, 2p4 right? That means valence electrons are present in the second shell. And there are no diorbitals present in the shell. Whereas the heavier members like sulfur or selenium they all have empty diorbitals available like 3D, 4D, 5D and so on. So they can basically expand their octet beyond 4 by using these empty diorbitals. For example, sulfur can easily form SF6 by expanding its octet or by using the empty diorbitals. But when oxygen combines with fluorine you will end up with something like OF2. You cannot expect something like OF6 as you observed in the case of SF6. Now even though we say that the maximum covalency of oxygen is restricted to 4 it practically never even exceeds 2. Except probably when it forms hydronium ions H3O+, where you can see that there are two covalent bonds between oxygen and hydrogen and a coordinate covalent bond where it has donated its lone pair of electrons to a proton. So this is one of the instances where oxygen extends its covalence beyond 2. Let's now discuss the consequence of small size and high electronegativity of oxygen. Because of the small size and very high electronegativity of oxygen it can actually form strong hydrogen bonds. And this is what we observe in water. Because of the high electronegativity difference between hydrogen and oxygen these atoms acquire partial charges like hydrogen acquires partial positive charge and oxygen acquires partial negative charge. And because oxygen also has the presence of two lone pair of electrons these can further coordinate with the hydrogen of another water molecule. Whereas if you compare it with H2O you can see that H2O is actually a gas and like H2O which is a liquid. This is because there is no such hydrogen bonding possible in H2O because of the larger atomic size of sulphur and lesser electronegativity as compared to oxygen. So while the molecules of water associate with each other through strong hydrogen bonding no such association is possible in H2O which is why it is a gas. Now something similar existed in the case of nitrogen too if you recall. Nitrogen again is the smallest one in its family group 15 and has the highest electronegativity as compared to the rest of the family members. And because of that nitrogen also had the unique ability to form hydrogen bonds. But nitrogen forms weaker hydrogen bonds as compared to oxygen because of its larger size and lower electronegativity as compared to oxygen. So to conclude what all did we learn in this video? Well we talked about the oxidation state of oxygen and the cases in which the electronegative oxygen can show positive oxidation states. We also talked about atomicity of oxygen and how it affects the physical properties like melting point and boiling point. We also saw how the absence of D orbital restricts the covalency of oxygen to 4. And more importantly we discussed how oxygen has the ability to form strong hydrogen bonds. Something that is although not very unique to oxygen but is definitely of great importance.