Nuclear decay and mass defect in nuclear chemistry.pptx

For Chemical Engineering

1. How do they decay? If N/P is high: Decay in order to reduce N/P 1. β – emission or Beta minus emission Example: C-14 decay to produce N-14 2. Neutron emission Two types of unstable nuclei Nuclei with appreciably higher N/P ratio Nuclei with significantly lower N/P ratio = anti neutrino

2. ( B) If N/P is too low: Decay in order to increase N/P Positron emission or Beta plus emission Example: N-13 decay to form C-13 Orbital/K-electron capture : Proton in the nucleus capture a K shell electron to form a neutron and emit a neutrino. Example: 3. Proton emission : An excited (due to β decay) nucleus may emit a proton *Very rare = neutrino

3. 3 Radioactivity Spontaneous emission of high energy particles from unstable nuclei Spontaneous emission of fundamental particle or light Nuclei falls apart without any external stimuli Discovered by Becquerel (1896) Extensively studied by Marie Curie and her husband Pierre (1898  early 1920's) Initially worked with Becquerel Radioactivity  is the spontaneous emission of radiation in the form of particles or high energy photons resulting from a nuclear reaction. Radioactivity is not influenced by heat, light, physical state or chemical combination.

4. 4 Fun Facts Marie and Pierre Curie discovered polonium and radium Nobel Prize in Physics 1903 For discovery of Radioactivity Becquerel, Marie and Pierre Curie—all three shared Nobel Prize in Chemistry 1911 For discovery of Radium and its properties Marie Curie only Marie Curie - first person to receive two Nobel Prizes and in different fields

5. 5 Radioactivity continued… Initially able to observe three types of decay Labeled them  ,  ,  rays (after first three letters of Greek alphabet) If they pass through an electric field, very different behavior

6. 6 Discovery of Radioactivity  rays attracted to negative pole so its positively charged  rays attracted to positive pole so its negatively charged  rays not attracted to either so its not charged   

7. 1. Alpha rays It is not a ray but consist of alpha particles which is merely the nuclei of helium atom. It’s velocity is 1/10 th the velocity of light. They ionize the gas through which they pass. They cause luminescence. They can not easily penetrate through solid matter but can penetrate mica, alumina etc.

8. 2. Beta rays These are also not rays but consist of negatively charged particles. It’s velocity is 10 times the velocity of α -particles. They ionize the gas through which they pass, but the effect is much lesser than that of α -particles. They produce less luminescence when allowed to fall on ZnS . Their penetrating power is 100 times that of α -particles.

9. 3. Gamma rays They are similar to X-rays and light rays, but wavelength is extremely smaller (10 -10 to 10 -13 m). They are harmful to living tissues. Their ionizing power is very low. They have very less luminosity They have high penetrating power, even greater than that of X-rays,10 times that of β -particles.

10. Comparison radioactive α β γ

11. Types of radioactive decay α - decay The nuclear disintegration process that emits alpha particles is called alpha decay. An example of a nucleus that undergoes alpha decay is uranium-238. In this nuclear change, the uranium atom (U-238) transmuted into an atom of thorium (Th-234) and, in the process, gave off an alpha particle. The alpha particle has two protons and 2 neutrons in it which were lost by the uranium atom. Thus the mass number of the daughter nucleus is obtained by subtracting 4 from that of the parent nuclei. Another alpha particle producer is thorium-230.

12. β -decay A beta particle is simply a high energy electron that is emitted from the nucleus. We will treat beta decay as a neutron splitting into a proton and an electron. The proton stays in the nucleus, increasing the atomic number of the atom by one. The mass number remains the same. The electron is ejected from the nucleus and is the particle of radiation called beta. the nuclear symbol representing an electron (beta particle) is Thorium-234 is a nucleus that undergoes beta decay.   

13. γ -decay Frequently, gamma ray production accompanies nuclear reactions of all types. In the alpha decay of U-238, two gamma rays of different energies are emitted in addition to the alpha particle. γ -decay does not change atomic no. and mass no. Most of the nuclear reactions also emit gamma rays, but for simplicity the gamma rays are generally not shown. Nuclear reactions produce a great deal more energy than chemical reactions. Chemical reactions release the difference between the chemical bond energy of the reactants and products, and the energies released have an order of magnitude of 10 3 kJ/mol. Nuclear reactions release some of the binding energy and may convert tiny amounts of matter into energy. The energy released in a nuclear reaction has an order of magnitude of 10 18 kJ/mol. That means that nuclear changes involve almost  a million times more energy  per atom than chemical changes!

14. Units of radioactivity 1 curie = 3.7x10 10  radioactive decays per second  [ dps ]. In the International System of Units (SI) the curie has been replaced by the  becquerel  ( Bq ), where 1 becquerel = 1 radioactive decay per second   = 2.703x10 -11   Ci . Rutherford (Rd): It is the amount of radioactive matter which undergoes 10 6 disintegrations per second. 1Ci = 37000 Rd

15. Decay Scheme The  decay scheme  of a radioactive substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships. In a decay scheme energy is represented on the Y-axis and atomic number on the X-axis. The arrows indicate the emitted particles. For the gamma rays (vertical arrows), the gamma energies are given; for the beta decay (oblique arrow), the maximum beta energy. For radioactive decays, the daughter nuclide is drawn lower than the parent as there is a loss of energy during the decay. If the nucleus of the daughter nucleus has energy levels, these are drawn above the stable state. ------------------------------------------------ Lines drawn between the parent nuclide, the daughter nuclide and any of its energy levels indicate the type of decay that occurs.

16. Example: Radioactive decay scheme for 60 Co 27   60 Co decays by emitting an electron (beta decay) with a half-life of 5.272 years into an excited state of  60 Ni, which then decays very fast to the ground state of  60 Ni, via two gamma decays. For more examples and details refer https://en.wikipedia.org/wiki/Decay_scheme

17. Standard practice (often not followed) Electron emission (beta minus) will  increase  the atomic number of the material and is denoted by a green arrow . Electron capture is a red arrow . Positron emission (beta positive) is a pink arrow Alpha decay is a double purple arrow Photon emission is a blue arrow

18. Mass Defect & Binding Energy Mass Defect : The difference between the sum of the masses of protons, neutrons and electrons present in an isotope of an element and the actual mass of the isotope. Let M’ be the theoretical mass of an isotope, m p be the mass of a proton, m n the mass of a neutron, m e the mass of an electron, A the mass number and Z the atomic number, then; M’ = Zm p + Zm e + (A-Z) m n M’ = Zm H + (A-Z) m n where m p + m e = m H & m H is the mass of a hydrogen atom. The actual mass of the isotope obtained from mass spectrograph is M, then Mass defect = M’ – M = Δ M i.e. Δ M = Zm H + (A-Z) m n - M Aim: Define binding energy of a nucleus. Explain the stability of the nuclei with the help of binding energy curve. Solve problems to compute binding energy

19. The quantity Δ M represents the loss of mass in the formation of the nucleus and this mass is released as energy as per Einstein’s mass-energy relationship. The energy released in the formation of the nucleus from its constituent nucleons is called binding energy of the nucleus. This is also the energy required if we want to break up the nucleus into its component nucleons. since Δ M is in a.m.u ., and 1 a.m.u . = 931.5 MeV , the binding energy = Δ M x 931.5 MeV The binding energy of a nucleus when divided by the number of nucleons gives the mean binding energy per nucleon . The greater this value greater will be the stability of the nucleus.

20. 21 Divide binding energy E B by mass number, E B / A Get binding energy per nucleon Binding Energies per Nucleon

21. 22 Implications of Curve Most E B / A in range of 6 – 9 MeV (per nucleon) Large binding energy E B / A means stable nucleus Maximum at A = 56 56 Fe largest known E B / A Most thermodynamically stable Nuclear mass number ( A ) and overall charge are conserved in nuclear reactions Lighter elements undergo fusion to form more stable nuclei

22. 23 Implications of Curve Fusion Researchers are currently working to get fusion to occur in lab Fission Heavier elements undergo fission to form more stable elements Reactions currently used in bombs and power plants ( 238 U and 239 Pu) As stars burn out, they form elements in center of periodic table around 56 Fe