Atoms And Their Isotopes

[Music] all matter is made up of atoms or molecules for example water each water molecule consists of two hydrogen atoms and one oxygen atom an oxygen atom has eight electrons traveling around the nucleus the nucleus consists of elementary particles called nucleons of which there are two kinds protons positively charged particles and neutrons which have no charge at all the oxygen nucleus normally contains eight nucleons of each type since positive and negative charges balance each other the whole atom is electrically neutral now what force holds the nucleons together let us consider a single example hydrogen in approximately every 7,000 hydrogen atoms one atom is of a particular type in addition to the proton its nucleus contains a neutron for this reason it is called a heavy hydrogen nucleus consider a proton and a neutron some distance apart the particles cannot exert electrical forces on one another because the neutron has no charge therefore they follow their own paths without affecting each other it is only if they come very close together but suddenly an attractive force is set up this is the so called nuclear force the exact behavior of which is still unknown suppose our normal hydrogen nucleus approaches a heavy one as is well known bodies with like charges repel each other thus to bring them together an external force must be applied at small distances the repulsive force is tremendous but again if the nuclei come very close together the nuclear force suddenly starts acting this force is so great that it easily overcomes the repulsive force of the protons the new particle thus formed is the nucleus of a special type of helium which occurs very rarely in only one atom of about 800,000 a normal helium nucleus has two neutrons summing up there are two types of helium nuclei the light nuclide and the normal nuclide and two types of hydrogen nuclei the normal and the heavy nuclide the heavy hydrogen nuclide is called deuterium there is also a third type of hydrogen which contains two neutrons and occurs in extremely small quantities it is called tritium but what of oxygen we have already seen the structure of a normal oxygen atom but one atom in about three thousand contains nine neutrons and one atom in about 500 has as many as 10 neutrons all these atoms contain eight protons and eight electrons therefore oxygen is given the atomic number eight it is this number which determines the chemical behavior the difference between the three oxygen nuclides is in the total number of nucleons that is 16 or 17 or 18 the three nuclides are called isotopes of oxygen Oh 16 Oh 17 and Oh 18 these isotopes are chemically identical in each of the two isotopes of helium are two protons consequently they both have the atomic number to the three hydrogen isotopes have the atomic number one now we can arrange the different nuclides according to the number of protons and neutrons they contain the simplest nuclides are the three isotopes of hydrogen each has one proton and zero one or two neutrons respectively so that the total number of nucleons is either 1 2 or 3 the next nuclides in the table are the two isotopes of helium with 2 protons each and either 1 or 2 neutrons the place where the single neutron belongs is beneath hydrogen 2 next to helium is lithium and so on oxygen as we have seen has three isotopes Floreen the next element has only one continuing we come to heavier elements chromium for example has the atomic number 24 and for isotopes the table ends with uranium which has the atomic number 92 there are three isotopes of uranium of which u-238 occurs most frequently the u-238 nucleus contains 146 neutrons and 92 protons now the nuclides can be divided into two groups the larger group shown in black contains the so-called stable nuclides the smaller group shown coloured contains the unstable or radioactive nuclides there are also great many other radioactive nuclides which can be made artificially to understand what is meant by the words stable and radioactive we must take a closer look at the elementary particles which make up the atom the electron and the proton have equal electric charges of opposite sign the neutron has no charge another important property of a particle is its mass the mass of the electron is very small we will give it the symbol small m the proton is much heavier large m the neutron is heavier than the proton but the difference is only two and a half times the mass of the electron now let us imagine a free Neutron one which is not bound inside a nucleus the neutron splits spontaneously into a proton and an electron it is important to study this reaction carefully the charges of a proton and an electron are equal and opposite matching the zero charge of the neutron but what about mass since we have only one electron we can see that 1 point 5 electron masses have disappeared in fact this mass has been converted into energy the kinetic energy of the electron this is a demonstration of Einstein's law that mass and energy have an equivalence and can be converted into each other according to this formula relating energy mass and the velocity of light disintegration of a neutron can take place in several nuclei let us consider tritium one of the neutrons is transformed into two particles a proton which stays in the nucleus and an electron which flies off the energy set free in this reaction is used partly to overcome the force of repulsion between the protons whilst the remainder is given to the electron in the form of kinetic energy comparing the two states it is seen that the nucleus at the left has one proton and is therefore an isotope of hydrogen the nucleus on the right has two protons and thus cannot be a hydrogen isotope in fact it is the light helium isotope helium-3 both of these reactions that of tritium and that of the neutron itself may be shown by means of equations similar to the for chemical reactions for historical reasons electrons emitted in this way are called beta particles these kinds of nuclear reactions are called electron decays or beta decays the beta decay of the neutron and the tritium nuclide can also be shown in the table the neutron decays to hydrogen and tritium to light helium generally a beta decay is represented by one step to the left and one step upwards because one Neutron is lost and in its place one proton is gained the next nuclide showing beta decay is beryllium ten its decay product is boron ten there are isotopes of carbon potassium rubidium indium and also of some very heavy nuclides all these nuclides belong to the small group of unstable or radioactive nuclides it may now be clear why distinction was made between the two groups as its name implies a radioactive nuclide decays spontaneously whereas a stable nuclide can be changed only by external forces a substance being radioactive does not mean that its decay is instantaneous in any large number of neutrons some will decay immediately while others will take longer decay takes place at a speed determined by the half-life of the substance the half-life for neutrons is twelve minutes this means that after 12 minutes 1/2 of the neutrons will have decayed and after another 12 minutes one half of the remaining neutrons and so on thus radioactivity gradually decreases with time every radioactive nuclide has its own specific half-life varying from a fraction of a second to billions and billions of years it may well be asked why some nuclides are stable and others are not to understand this let us take any nucleus and see what forces are at work first there is the nuclear force holding the nucleons together this force is opposed by the repulsive forces of the protons therefore at first sight it would seem that a nucleus would be most stable if it contained only neutrons such a nucleus does not exist in nature but if it did it is easy to see what would happen one Neutron after another would decay by beta emission simultaneously the repulsive forces of the protons would grow making further beta decays more and more difficult finally an equilibrium would be reached in which the protons already present prevent the creation of new ones in the table a nuclide of neutrons only would be located on the baseline after successive beta decays a stable position would be reached now all equilibrium positions shown in black form the narrowband occupied by the stable nuclides this band separates two regions one in which there are too many protons and the other in which there are too many neutrons if a nuclide is located in the latter region one or more of the neutrons will show beta decay as we have seen with the radioactive nuclides already discussed the other region contains only three nuclides one of these three is an isotope of beryllium this isotope has an excess of one proton this proton can be transformed into a neutron with the aid of an electron which is captured from the inner electron shell of the atom or the K shell this process is called electron capture or K capture in the case of beryllium this K capture is followed by the emission of an electromagnetic wave called a gamma ray in the table K capture is indicated by one step to the right and one step downwards because one Neutron is gained at the expense of one proton summing up again there is a narrow band which contains stable nuclides and which separates two regions containing radioactive nuclides in one of these regions neutrons predominate and beta decays occur in the other region that is an excess of protons which gives rise to K capture these processes are accompanied by gamma emission on many but not all occasions with heavy nuclides for example uranium-238 a third type of radioactivity occurs this uranium isotope can emit a helium nucleus followed by the emission of a gamma ray since the nucleus loses two protons and two neutrons this is represented in the table by two steps to the left and two steps downwards helium nuclei are called alpha particles the exact mechanism of emission of alpha particles is not fully understood since the behavior of the nuclear force is still obscure however it appears that the nuclear force builds up to some form of saturation so there might be a cluster of two protons and two neutrons wandering around the nucleus and influenced by the forces of repulsion of the other protons these repulsive forces can be so great especially in heavy nuclei that the alpha unit is ejected inspection of the table shows that the decay product of uranium-238 is thorium-234 but this nuclide is also radioactive two successive beta decays will take place giving uranium 234 which will in turn emit an alpha particle and so on until lead-206 is formed which is no longer radioactive a sequence of radioactive nuclides like the one just discussed is called a radioactive chain there are three of these chains they start with uranium-238 uranium-235 and thorium-232 it may well be asked how it is that radioactive nuclides exist at all since they tend to decay towards a stable state many radioactive nuclides were formed at a time when cosmic conditions differed greatly from today due to their enormous half-lives some of them have survived in the Earth's crust others are being produced from the decay chains of uranium and thorium and today the light radioactive isotopes are being created continuously in the upper layers of the atmosphere where the nuclei are broken up by an endless stream of cosmic rays [Music]