 Welcome to the second lecture on actinides. So, in the previous lecture we have discussed about the actinides and why it is important to study the chemistry of actinides and also some of the manmade actinides I have given a detailed account of neptunium and plutonium synthesis. Now in this lecture we talk about some of the other aspects like actinides in nature. So, though thorium, protactinium and uranium and are available in the nature in large quantities, some of the other actinides which are also available in nature very-very trace levels are actinium and plutonium and surprising that plutonium also is available in ultra trace levels. As discussed in the previous lecture, this plutonium comes from some of the piston reaction taking place in nature where in the very large deposits of uranium is there in the uranium mines like the Okulofenomena where the mine in Gabon, Africa. Now, there as I have already discussed, missile content of the uranium was very high. Now in earth crust at this moment we have around 0.03 percent uranium, the average value I have given here and in seawater uranium is 3 ppb, very-very low concentration of uranium. Nevertheless, it is a rich source of uranium if somebody can really tap uranium from the seawater. Now, compared to uranium, thorium concentration is much higher in the earth crust. So, thorium in earth crust is around 0.1 percent, but in the seawater thorium content is very less, it is probably because of the hydrolysis of thorium which is existing in the titraruland oxidation state. Now, coming to the natural uranium we have the fissile content that is 235 uranium is 0.72 percent and other uranium sources also can be defined here as depleted uranium where the fissile content or the 235 uranium is less than 0.72 percent, low enriched uranium 0.72 to 20 percent 235 uranium, highly enriched uranium 20 to 90 percent 235 uranium and the weapon grade uranium is greater than 90 percent of 235 uranium. As I mentioned this 239 plutonium which is very ultra-trash level is reported in this uranium natural reactors that is the oklu phenomena where the 235 contain at that phase when this reactor was suggested to be operative was more than 3 percent 235 in the natural uranium in the mine and oklu. Now, 244 plutonium was detected in rare earth mineral bascinocyte and also very very minute concentration that is one part in 10 to the power 11. Now, some of these other actinides like protactinium 231 this can be formed by some of the reactions as is written here. Similarly, 226 radium which with a subsequent neutral capture it gets 227 radium and the 227 radium can go to 227 actinium. So, this is how actinium 227 is formed and this protactinium 231 is formed from thorium 230 which by Newton capture it goes to 231 thorium and then 231 thorium decays by beta decay to 231 protactinium. So, this is how protactinium is formed and also actinium is formed to 27 actinium. Another isopropop protactinium which is very important is 233 protactinium it is obtained by irradiating 232 thorium by neutrons and you get 233 thorium which has a 22.3 minutes half like and by beta decay it gives 233 protactinium which again another beta decay it gives 233 uranium 233 uranium is very very important again for reactors which are operating on uranium 233 as the fissile material. Now, when this reaction is taking place there is also the possibility of uranium 232 formation which is formed by different reactions as given here like from 232 thorium it can form 233 protactinium which by beta decay goes to uranium 233 as I have already mentioned, but this 233 uranium can also undergo a N2N reaction to give 232 uranium. Alternatively we also can have this protactinium 233 the same reaction, but instead of decaying before decaying to 233 uranium it can also undergo N2N reaction to give 232 uranium. Thorium 232 also can undergo N2N reaction and it gives 231 thorium which by beta decay gives protactinium 231 and which can capture a neutron and by N gamma it goes to 232 protactinium which undergoes beta decay to give the 232 uranium. So, this is how actually this uranium 232 is formed in a rather small quantity while reducing this 233 uranium, but it is very important because this uranium 232 has to be removed from the uranium 233 otherwise the operation becomes very difficult as you can see from this decades reactions shown here from uranium 232 goes to 228 thorium then 224 radium to 20 radon to 16 polonium 212 lead and these are all by alpha decay and then to a 2212 lead by beta decay it goes to 212 bismuth which undergoes another alpha decay to give 208 thorium which is a very hard gamma emitter and this gives significant dose to the working personnel. Now as I mentioned this other actinides also can be synthesized in the reactor by neutron capture we have just mentioned about how protactinium is formed in the reactor also I will show here how neptunium 239 is formed in the reactor by irradiating 238 uranium by a neutron which then is produces 239 uranium and it undergoes a beta decay to give 239 neptunium which has 2.3 days half life and it is converted to 239 plutonium again by a beta decay this plutonium 239 can undergo alpha decay to give 235 uranium so this is a general decay series of this 238 uranium if it is undergoing a neutron capture also this 238 uranium can undergo neutron capture to give 237 uranium by a n2n reaction and which again decays by beta decay to 237 neptunium 237 neptunium is very very important because it has a very long half life and once it is produced in the reactor it goes to mostly to the waste as a minor actinide it has a very significant repercussion in the radioactive waste management because of the very long half life of 2.1 into 10 to the power 6 years 237 neptunium is also produced in the reactor by 235 uranium capturing a neutron giving 236 uranium which is another neutron capture gives 237 uranium and by beta decay it gives 237 neptunium so these are the waste this 237 neptunium is produced also some of the relatively heavier actinides like americium and curium they are also produced by again neutron bombardment like 239 plutonium with four neutrons it gives 243 plutonium and then by a beta decay it gives 243 americium which can capture a neutron to give 244 americium and which again undergoes beta to give 244 curium so this is how americium and curium this are also produced in the nuclear reactors now for high yield we need actually high neutron flux as i have already mentioned in the previous lecture this thermonuclear explosion it was taking place and there you we have detected angstanium and fermium that is how these elements were discovered and also if you take 239 plutonium and you irradiate in a reactor with a very high neutron flux like 3 into 10 to the power 14 neutrons per centimeter square per second then you need a very years of irradiation for producing even 1 milligram of californium 252 i have given here some of the actinides which are produced in a reactor by irradiation you can see that 248 curium this is formed with 150 milligram 248 curium is formed and 249 berculeum is formed only 50 milligram under the same conditions and 252 californium you get 500 milligram 253 angstanium only 2 milligram and 257 fermium only 1 picogram so this is because of this reaction cross sections are also less and also the t half of this radionuclides or the actinides you can see here it is very very less so that is how it is decaying also a 248 curium it was 10 to the power 5 years whereas for angstanium it is only 20 days and for fermium it is only 100 days so that is how it decays also the heavier actinides and the amount also is less because of the very low neutron capture cross sections there are also the actinide synthesis can be done by charge particle bombardment for example 238 plutonium is synthesized by deuteron bombardment of 238 uranium the reaction is given here where take 238 uranium and the deuteron you get 238 neptunium and this 238 neptunium undergoes a beta decay with a 2.1 days half life 238 plutonium this is a reaction actually which was used as I mentioned in the previous lecture for plutonium production for the first time in the Berkeley radiation laboratory now similarly this californium also can undergo a charge particle bombardment like 11 boron pi plus with this ion beam if it is bombarded then it gives 256 lorensium and also four neutrons are emitted this is how the heavy actinides and the trans actinides are made by the bombardment of charge particle this will cover in a more detail when I am discussing about the trans actinides now I will summarize here this heavier actinide elements synthesis by accelerators so what we do is these accelerators they should have good beam current and high energy charge particles are needed the product many times is contaminated by the fission product with a thicker fission reaction will be taking place we are much larger extent than this production of the heavier actinides so that is how this separation of this heavier actinides from the fission product is required also if you go to very heavy actinides or even trans actinides their atom at a time scale these elements or the nucleates are formed so that is how these continuous experiments are needed if you want to carry out some of these experiments particularly the chemistry experiment online experiments has to be done when they experience this at an atomic time scale these actinides are produced and this has to be transported to the adjacent laboratory where this experiment has to be carried out and there it is continuously these experiments needs to be carried out because otherwise they are decaying and this identification is done by parent daughter and the granddaughter correlation where you have one particular actinide A which is decaying to B and which is decaying to C and A because of it has a very short half life you when you carry out the experiment you cannot probably see A but B and C will be in significant quantities so by detecting B and C you can always say that well A has also formed and that is how the chemistry of A has been established the products are also highly radiative because of their very small half lights and because of that the solutions are damaged due to radiolysis these have very high radio toxicity and also because of this remote handling is a requirement for this type of experiments to be carried out identification as I said it is difficult because you have a host of fission products also are formed so identification is difficult and you need to separate from the fission products and then identify these radio nucleates the products also have a very low stability as I have already mentioned nobellium isotope as you can see here even nobellium with a half life of one hour or lordship with a half of three minutes they are isolated but then because of the low stability we have to carry on the chemistry very very fast now coming to the position in the periodic table in the late 1930s only four actinides were known that is the actinium thorium protactinium and uranium and the last three were placed in the periodic table as another transition series that is the 6d transition series as the homologues of aphenium tantanum and tungsten now I have already mentioned in the chemistry of neptunium how the neptunium was placed under rhenium so because of this particular positioning in the periodic table to the 6d transition series this was suggested that time however the quantum theory of Bohr and other experimental reasons suggested that these actinides instead of being the 6d series they may be the 5f series and also the inconsistency between the theory and chemical properties made it difficult in placing them in the periodic table so there are a lot of confusion I will be showing in the next slide how this confusion was actually cleared then Alfred Werner in 1905 suggested that thorium as the homologue of cerium because thorium and cerium they are chemistry very much comparable and he showed the seed for a new series like the lanthanides after the discovery of neptunium and plutonium and based on their chemical properties their placement in the 4th transition series was challenged and it was concluded that neptunium and plutonium they are chemistries more similar to that of uranium rather than that of rhenium and osmium so after the discovery of americium and curium by seborgs group in 1944 the similarity between lanthanide and actinide was recognized because americium and curium both PA was equivalent metal ions and the similarity in the spectroscopic and magnetic properties also was due to the similarity in the electronic configuration I will be discussing the electronic configuration shortly similarly in the crystallographic properties when you are near matching in the ionic ready of these actinides to that of the lanthanides also suggested that they may be similar to the lanthanides oxidation states however are not similar to that of the lanthanides like thorium protactinium and uranium they are not tripositive in solutions but they are plus four plus five and plus six oxidation states are formed in those cases variable oxidation states for the early actinides also have been detected but then the reason for this will be because of the 7s 16 and 5 wave energy logins are very close in the energies so that is how the variable oxidation states of the actinides were explained now as I was mentioning there were a lot of confusion actually above the 5f series before the 1940s so this is some of the I will list down how these different groups they were proposing where the 5f series will be 1913 read bar proposed that transition groups should be there around uranium with five of electrons being filled so lanthanide type of series but beyond uranium 1923 more suggested that 5f series should start with element 94 and in 1924 Goldsmith he proposed that up to 96 should be homolog of the platinum group and beyond that only you can have the 5f series in 1926 Sugira and Viray they have done calculations and indicated that the first 5f electron entry should be for element 95 in 1933 who involved Goldsmith did more refined calculations and suggested that the first 5f electron filling should be not for element 95 but for element 93 in 1926 Macmillan, Mollay and Smith they have suggested that a 5f cell should start with thorium and in 1926 again Sweeney he suggested that five of electrons should start with protactinium and uranium in 1934 Saha and Saha they suggested the 5f electron should start at thorium and in 1930 Carapetov suggested the first 5f electron with element number 93 so as you can see here that many people suggested that it should be at 93 or 94 and some even 95 subsequently 1928 one cross suggested that again it will be starting from element 92 that is uranium and 1938 Quill suggested that it starts from 95 or 99 so he has increased to the heavier 89s 1937 Goldsmith he changed his original view and based on the crystallography work suggested that the 5f electrons enter from protactinium or maybe thorium or uranium so he suggested the name attenite and also he suggested other alternative names like thorite, iranite or protactinite now what is the major objection coming that this nectonium and plutonium they behave like uranium and thorium but not like rhenium and osmium which is there in the periodic table for the 6d series now there is no evidence for the volatile plutonium tetraxide in contrast to the volatile osmium and ruthenium tetraxides and also there is no evidence for an oxidation number of eight in case of plutonium so they suggested that this plutonium is definitely not behaving like osmium the observation of jacaric acid or of the isomorphism of the compounds this thorium dioxide, iranium dioxide, neptonium dioxide and plutonium dioxide they found that it is not isomorph, iranium dioxide is not isomorphic to molybdenum dioxide and his observation of the regular decrease in the radius of the metallic ion in these oxides also suggested that this is a separate series similar to the rhenthonite series other evidence is the magnetic susceptibility of iranium and plutonium sharpness of the optical objection of iranium and plutonium evidence of organic complexes of iranium 4 plus and plutonium 4 plus also the analysis of the spectrum of the iranium atom come to the conclusion that the electron configuration of the lowest state of iranium is pi wave 3 6d 7 h 2 with the term symbol 5 l 6 similar to the lanthanide series the electron does not go to the 60-orbiton but to the 5f orbital now the plus 4 state was prevalent for thorium iranium and neptonium and plutonium the last three under the reducing condition and also plus 3 state was also reported for iranium neptonium and plutonium under the suitable reducing conditions actinide series differed from the lanthanides as the higher oxidation states prevailed in case of the lighter actinides and thorium behave like cerium as both of the plus 4 oxidation states a with this background and also the discovery of americium and turium by cibor and their chemistry studies suggested that they were more like the lanthanides rather than like the transition elements so cibor was tempted to propose this actinide concept and he has sent a publication that time suggesting his actinide concept and his colleagues they advised against this because he said that this is a very wild idea you cannot have a actinide series similar to the lanthanides as cibor was proposing he said that your reputation will be ruined but cibor said that i didn't have that much reputation at that time and i was also more fortunate that i was right ultimately he was proven right that there was a actinide series similar to the lanthanides now coming to the position in the periodic table as i have mentioned before this the actinides initially proposed to be under this force transition series it was proposed however after cibor's proposition this was the actinide series where thorium is also just below cerium and protecinium below prasidium however this behavior of the lanthanides was entirely different because they are all plus three oxidation state and for actinides only beyond americium you have the plus three oxidation state now coming to differences between the actinides and lanthanides lanthanides are naturally occurring except promethium and actinides man made except for actinium thorium protectinium and uranium and there is a difference in energy between five wave and 60 orbitals of actinides are less than that of the 4f and 5d orbitals of the lanthanides five of orbitals of the actinides have greater spatial extension and hence participate in the bonding some cases even covalent bondings are reported for the actinides and that is the basis of the lanthanide actinide separation which i will be discussing in a future lecture now coming to this electronic configuration of the actinides as i have shown here this from starting from actinium to laurencium you see the electronic configurations and also i have for comparison purpose i have put the electronic configurations of the lanthanides you can see here that for lanthanides only for cerium, gadolinium and mutasium you have the d electron and for all others you find that the f electrons are getting filled on the other hand for the actinides you have you see that the initial actinides up to neptunium you have the d electrons are also there and also the f electrons are also there now beyond neptunium you have this plutonium and americium there you do not have the d electrons and apart from curium you have all other actinides again with only the f electrons getting filled gradually so this is a similarity of the actinides with the lanthanides starting from plutonium but the early actinides that i have already mentioned because of the comparable energy levels of the 5 f 6d and the 7s levels so that is this type of configurations are possible now just coming to the electronic configuration also here you can see i have given this figure where you see that this comparison of f n minus 1 d h 2 and f n dash 2 these two electronic configurations are compared as you see here f n minus 1 d s 2 is more stable up to the element number 93 that is neptunium and beyond that like the plutonium you have this f n h 2 type of configuration is there plutonium and then americium and up to curium again you have this d f n minus 1 d h 2 type of configuration and beyond curium again you have this configuration where you have f n h 2 which is more stable so this is what i would like to cover here in this electronic configuration and we will discuss more about this chemistry of actinides in the subsequent lectures thank you