Isotopes , isobar, isotones

This presentation is about nuclear chemistry.

1. ISOTOPES , ISOBAR, ISOTONES

2. ISOTOPES The radioelements must exist in various forms different from each other by their masses and by their nuclear properties . About 40 atoms with different half-lives were discovered , for which 12 places in periodic table of elements. The atoms of an element which have same atomic number but different mass number or atomic masses.

3. Isotopes that emits radiation are called radioactive or radioisotopes . VELLAY OF STABILITY A nucleus is stable if it cannot be transformed into another configuration without adding energy from the outside . thousands of nuclides that exist, about 250 are stable. A plot of the number of neutrons versus the number of protons for stable nuclei reveals that the stable isotopes fall into a narrow band . This band called STABILITY BAND .

4. The lighter stable nuclei, in general, have equal numbers of protons and neutrons. For example, nitrogen-14 has seven protons and seven neutrons. Heavier stable nuclei, however, have increasingly more neutrons than protons . For example:iron-56 has30 neutrons and 26 protons, an n:p ratio of 1.15, whereas the stable nuclide lead-207 has 125 neutrons and 82 protons, an n:p ratio equal to 1.52. This is because larger nuclei have more proton-proton repulsions, and require larger numbers of neutrons to provide compensating strong forces to overcome these electrostatic repulsions and hold the nucleus together.

5. Figure 1 This plot shows the nuclides that are known to exist and those that are stable. The stable nuclides are indicated in blue, and the unstable nuclides are indicated in green. Note that all isotopes of elements with atomic numbers greater than 83 are unstable. The solid line is the line where n = Z.

6. The nuclei that are to the left or to the right of the band of stability are unstable and exhibit radioactivity. They change spontaneously (decay) into other nuclei that are either in, or closer to, the band of stability. These nuclear decay reactions convert one unstable isotope(or radioisotope)into another, more stable , isotope.

7. EXAMPLES OF CHLORINE ISOTOPE Isotopes  Atomic number Mass number  Electrons  Protons Neutrons  35Cl 17 35 17 17 35 - 17 = 18 36Cl  17  37  17  17  37 - 17 = 20

8. EXAMPLE OF NEON ISOTOPES. Neon has three isotopes with atomic number 10 and mass numbers, 20, 21 and 22. Isotopes Atomic number Mass number Electrons  Protons Neutrons 20 Ne 10 20 10 10 20 - 10 = 10 21 Ne  10  21  10  10  21 - 10 = 11 22 Ne  10  22  10  10  22 - 10 = 12

9. ISOBARS The word isobar means equally heavy is taken from the Greek isos =equal, and barys =heavy . The atoms which have same mass number but different atomic number are called isobars. Hence, as isobars have different atomic number, therefore, they have a different number of electrons, and their chemical properties are different.

10. Stability of isobars Stability of isobars d/p upon mass excess MASS EXCESS The mass excess of a nuclide is the difference between its actual mass and its mass number in atomic mass units .The mass number of uranium is 236 U, is 236. Because the actual mass is 236.045563 u, its mass excess is +0.045563 u.

11. Figure 2 Mass excesses for isobars vary as Atomic number Z vary. The stable isobar is Cs with lowest mass excess in amu that is -0.0870. MASS EXCESS OF SOME ELEMENTS IN amu and MeV,,,,,, Nuclide Mass  excess ( amu ) Mass  excess (MeV) In 49 -0.0896 -83.5 Sn 50 -0.0943 -87.8 Sb 51 -0.0958 -89.2 Te 52 -0.0967 -90.1 I 53 -0.0944 -87.9 Xe 54 -0.0915 -85.2 Cs 55 -0.0870 -81.0

12. ISOTONES Atoms which have same number of neutrons. Due to different mass number and atomic number, they show different chemical and physical properties.

13. “ The breaking of a large nucleus into smaller pieces is called fission” Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger binding energies per nucleon—that is, mass numbers and binding energies per nucleon that are closer to the “peak” of the binding energy graph near 56 . Sometimes neutrons are also produced NUCLEAR FISSION

14. DIAGRAM

15. Chain reactions of fissionable materials can be controlled and sustained without an explosion in a nuclear reactor Any nuclear reactor that produces power via the fission of uranium or plutonium by bombardment with neutrons must have at least five components: nuclear fuel consisting of fissionable material, a nuclear moderator, reactor coolant, control rods, and a shield and containment system FISSION REACTORS

16. Nuclear fuel consists of a fissionable isotope, such as uranium-235, which must be present in sufficient quantity to provide a self-sustaining chain reaction Nuclear reactors require a fuel with a higher concentration of U-235 than is found in nature; it is normally enriched to have about 5% of uranium mass as U-235. At this concentration, it is not possible to achieve the supercritical mass necessary for a nuclear explosion NUCLEAR FUELS

17. A nuclear moderator is a substance that slows the neutrons to a speed that is low enough to cause fission. Modern reactors in the US exclusively use heavy water ( 1 2 H2 O) or light water (ordinary H2O), whereas some reactors in other countries use other materials, such as carbon dioxide, beryllium, or graphite. NUCLEAR MODERATORS

18. A nuclear reactor coolant is used to carry the heat produced by the fission reaction to an external boiler and turbine, where it is transformed into electricity. Two overlapping coolant loops are often used; this counteracts the transfer of radioactivity from the reactor to the primary coolant loop REACTOR COOLANTS

19. Nuclear reactors use control rods to control the fission rate of the nuclear fuel by adjusting the number of slow neutrons present to keep the rate of the chain reaction at a safe level. Control rods are made of boron, cadmium, hafnium, or other elements that are able to absorb neutrons. Boron-10 CONTROL RODS

20. Reactors are equipped with a containment system (or shield) that consists of three parts: The reactor vessel, a steel shell that is 3–20-centimeters thick and, with the moderator, absorbs much of the radiation produced by the reactor A main shield of 1–3 meters of high-density concrete A personnel shield of lighter materials that protects operators from γ rays and X-rays SHIELD & CONTAINMENT SYSTEM

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