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Atomic theory


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Atomic theory

  1. 1. LecturePLUS Timberlake 1 Atomic Theory  Atoms are building blocks of elements  Similar atoms in each element  Different from atoms of other elements  Two or more different atoms bond in simple ratios to form compounds
  2. 2. LecturePLUS Timberlake 2 Subatomic Particles Particle Symbol Charge Mass Electron e- 1- 1/1840 Proton p+ + 1 Neutron n 0 1 Charge measure in terms of electronic charge: e=1.6x10^-19 Mass is measure in unified atomic mass: units(u) 1 u is 1/12 of the mass of a carbon-12 atom.
  3. 3. LecturePLUS Timberlake 3 Location of Subatomic Particles 10-13 cm electrons protons neutrons 10-8 cm nucleus
  4. 4. LecturePLUS Timberlake 4 Atomic Number Counts the number of protons in an atom
  5. 5. LecturePLUS Timberlake 5 Atomic Symbols  Show the mass number and atomic number  Give the symbol of the element mass number 23 Na sodium-23 atomic number 11
  6. 6. LecturePLUS Timberlake 6 Number of Electrons  An atom is neutral  The net charge is zero  Number of protons = Number of electrons  Atomic number = Number of electrons
  7. 7. LecturePLUS Timberlake 7 Subatomic Particles in Some Atoms 16 31 65 O P Zn 8 15 30 8 p+ 15 p+ 30 p+ 8 n 16 n 35 n 8 e- 15e- 30 e-
  8. 8. LecturePLUS Timberlake 8 Isotopes  Atoms with the same number of protons, but different numbers of neutrons.  Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 37Cl 17 17 chlorine - 35 chlorine - 37
  9. 9. LecturePLUS Timberlake 9 Learning Check Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 #p _______ _______ _______ #n _______ _______ _______ #e _______ _______ _______
  10. 10. Nuclear Density • Mass of proton m(p)=1.67 x 10^-27 kg • Radius of proton r = 0.80 x 10^-15 m • Find the density=
  11. 11. Isotopes Atoms made of protons, neutrons and electrons. Neutrons+ Protons: Nucleus • Electron + Proton # influences-> Chemical Properties • Protons+ Neutron # influences-> Physical Properties Isotopes Atoms with same number of protons but different number of protons are Isotopes.
  12. 12. Radioactivity • A stable nuclei emits radiations. Radiation was discovered by Henri Becquerel in 1896. These radiations are not influenced by any outside conditions such as temperature or pressure. • It is a spontaneous process, but it occurs at fixed intervals. Hence a proportion of sample decay will occur in that time interval.
  13. 13. Three particles present in these Radiations: • Alpha, Beta And Gamma Alpha Beta Gamma
  14. 14. • We know that nuclei consists of protons and neutrons, if the balance between these two types of particles is too far to one side, the nucleus may emit alpha or B radiations. • Gamma emitted after alpha or B radiation to release excess energy.
  15. 15. A third conserved quantity • In radioactive decay, both nucleon number A and proton number Z are conserved. • On comparison b/w mass before and after decay, we witness a decrease in mass. • This is because energy lost caries mass- recall E=mc^2, so we witness a net decrease in energy.
  16. 16. Properties of Ionizing radiation • On moving past atoms, alpha or B collide/rub with them and this as a result causes to knock or drag away electrons from an atom. • This process is called Ionization. • Now, Energy is lost during ionization. Size: Motion: Ionization Ability: • Alpha: more mass &charge | slow | Most • Beta: lighter | faster | Moderate • Gamma: no charge | Least
  17. 17. Lesson Contents 1. Physical properties of a, b and g 2. Penetrating power of a, b and g 3. N v Z graphs 4. Decay laws
  18. 18. Alpha Radiation Alpha particles contain two protons and two neutrons
  19. 19. Alpha Radiation a has the same constitution as a helium nucleus Alpha particles may be written as They have a double positive charge and a mass of 4 u
  20. 20. Alpha • Since most effective ionization therefore, most energy is lost during travel, hence cover least distance and in a cloud chamber, most prominent tracks are formed.
  21. 21. Detection of all three:
  22. 22. Beta-minus Radiation Beta-minus particles are electrons
  23. 23. Beta-minus Radiation b- is produced when a neutron decays Beta-minus particles may be written as They have a negative charge and a mass of 1/1800 u
  24. 24. Beta-minus Radiation b- is produced when a neutron decays The surplus mass is released as kinetic energy in the b- and as an antineutrino + Energy
  25. 25. Gamma Radiation Gamma rays are a form of electro- magnetic radiation
  26. 26. Gamma Radiation g release is often associated with a or b decay Gamma rays remove energy from an unstable nucleus
  27. 27. Type of radiation emitted & symbol Nature of the radiation (higher only) Nuclear Symbol (higher only) Penetrating power, and what will block it (more dense material, more radiation is absorbed BUT smaller mass or charge of particle, more penetrating) Ionising power - the ability to remove electrons from atoms to form positive ions Alpha a helium nucleus of 2 protons and 2 neutrons, mass = 4, charge = +2 Low penetration, biggest mass and charge, stopped by a few cm of air or thin sheet of paper Very high ionising power, the biggest mass and charge of the three radiation's, the biggest 'punch'! Beta high kinetic energy electrons, mass = 1/1850, charge = -1 Moderate penetration, 'middle' values of charge and mass, most stopped by a few mm of metals like aluminium Moderate ionising power, with a smaller mass and charge than the alpha particle Gamma very high frequency electromagnetic radiation, mass = 0, charge = 0 Very highly penetrating, smallest mass and charge, most stopped by a thick layer of steel or concrete, but even a few cm of dense lead doesn't stop all of it! The lowest ionising power of the three, gamma radiation carries no electric charge and has virtually no mass, so not much of a 'punch' when colliding with an atom
  28. 28. Penetrating power
  29. 29. Effect of Magnetic Fields
  30. 30. Decay laws - alpha When an isotope emits an a particle  Its nucleon number decreases by 4  Its proton number decreases by 2 For example:
  31. 31. Decay laws – beta-minus When an isotope emits a b- particle  Its nucleon number is unchanged  Its proton number increases by 1 For example:
  32. 32. Decay laws – beta-plus When an isotope emits a b+ particle  Its nucleon number is unchanged  Its proton number decreases by 1 For example:
  33. 33. Decay laws Try writing the nuclear equations for the decay of these isotopes
  34. 34. Decay laws Answers
  35. 35. Decays
  36. 36. Randomness and Decay: Spontaneous: • Decays not effected by the presence of other nuclei. • No chemical reaction, external factors like pressure and temperature effect the decay. • Random: • Impossible to predict the time • Each nucleus has an equal probability to decay