NUCLEAR STABILITY.pptx

Nucleus core

1. 1 “Why do protons stay together when positive charges repel each other ?” The main reason is because of a force called Strong Force . Opposes the electrostatic force.

2. Chemical Reactions New substances are made through the formation of new nanoscopic “units” by making and/or breaking chemical bonds (Dalton) All the “action” is outside of the nuclei Nuclei remain unchanged! Chemical Bonding involves moving electrons , not nuclei *

3. Nuclear Reactions It’s all about changing the nucleus! Independent of any “standard” chemical reactions *

4. NUCLEAR STABILITY & BINDING ENERGY SHIELA MARIE B. SIBAYAN ©

5. Two kinds of STABILITY One refers to whether a nuclide will undergo spontaneous nuclear decay . Does the nuclide decay (unstable, radioactive) or not (stable)? The “valley of stability” Kinetic Stability Thermo-dynamic Stability * One refers to how stable one nuclide is compared to another , in terms of “overall configuration of nucleons” Applies to all nuclides, radioactive or not Assessed by Binding energy per nucleon Ex. 206 Pb is a stable nuclide. 238 U is radioactive Ex. 56 Fe is more stable than 206 Pb or 2 H

6. 6 Nuclear Stability As a general rule, a nucleus will need a neutron / proton ratio of 3:2 (or 1.5:1) in order to stay together. This rule is more precise for larger nuclei.

7. 7 Nuclear Stability Of all known isotopes of natural elements (about 1500), only 250 of them are stable. All of these stable isotopes have an atomic number in between 1 and 83 .

8. Stability Curve Atomic number Z Neutron number N Stable nuclei Z = N 20 40 60 80 100 40 100 140 20 60 80 120 Nuclear particles are held together by a nuclear strong force. A stable nucleus remains forever, but as the ratio of N/Z gets larger, the atoms decay. Elements with Z > 82 are all unstable.

9. Atomic Mass Unit, u One atomic mass unit (1 u) is equal to one-twelfth of the mass of the most abundant form of the carbon atom--carbon-12. Atomic mass unit: 1 u = 1.6606 x 10 -27 kg Common atomic masses: Proton: 1.007276 u Neutron : 1.008665 u Electron: 0.00055 u Hydrogen: 1.007825 u

10. Mass and Energy The energy of a mass of 1 u can be found: E = (1 u) c 2 = (1.66 x 10 -27 kg)(3 x 10 8 m/s) 2 E = 1.49 x 10 -10 J Or E = 931.5 MeV When converting amu to energy:

11. Mass Defect The nucleus is composed of protons and neutrons the actual mass is less than the mass of the separate particles. The "missing" mass is in the form of energy holding the nucleus together.

12. Mass Defect Mass defect (M.D) is another way of saying nuclear B.E. It is simply the nuclear B.E. expressed not as MeV but in mass units (MeV/c 2 ) = Mass constituents of atom – mass of atom

13. Uranium-238 238 U 92 A proton is 1.00728 amu A neutron is 1.00867 amu atomic number (protons) nucleons (protons & neutrons) The nuclear mass of uranium-238 is 238.0003 amu

14. Uranium-238 (92)(1.00728) = 92.6698 (146)(1.00867) = 147.2658 92 protons 146 neutrons predicted mass = 239.9356 actual mass = 238.0003 mass defect = 1.9353 amu

15. Binding Energy of 238 U predicted mass 239.9356 actual mass 238.0003 1.9353 amu D E = D mc 2 = 931.5 MeV/amu What is the binding energy per nucleon of 238 U?

16. Binding Energy of 238 U Binding energy per nucleon = (1.9353 amu)(931.5 MeV/amu) 238 nucleons = 7.57 MeV

17. Binding Energy The mass defect indicates the total energy involved in holding the nucleus together. To determine the stability of the nucleus, the binding energy per nuclear particle is a better measure.

18. The binding energy of an atom is the energy released as all the constituent particles (n, p and e) come together FROM INFINITY under both the STRONG force and the EM force. The binding energy is something that is LOST from the atomic system. Thus it is not something that the system possesses. BINDING ENERGY

19. SINGLE NEUTRON SEPARATION ENERGY The same method can be used to easily compute the “Single Neutron Separation Energy” – which is the energy required to “pull” a neutron out of the nucleus.

20. SINGLE PROTON SEPARATION ENERGY The same clever strategy applies to finding the “Single Proton Separation Energy” S p . But note here there is a difference – we must be careful in counting electron mass. [Mass of Final Products – Mass of Initial atom] c 2

21. Nuclear Binding Energy Energy must be added to a nucleus to separate it into its individual nucleons (protons and neutrons). The energy that must be added to separate the nucleons is called the binding energy E B . The binding energy is the energy by which the nucleons are bound together.

22. How much is electronic binding energy? There are two types of binding energy in the atom – Strong Nuclear B.E. and the Electromagnetic B.E. of the electrons to the nucleus.