Dual nature of matter; Atom and Nuclie

Dual Nature of matter and radiation

  1. J Thomson predicted that the cathode rays are nothing but the stream of fast moving electrons.
  2. The number of photoelectrons emitted per second is directly proportional to the intensity of the incident radiations.
  3. Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron is independent of the intensity of the incident light but depends only upon the frequency of incident light.
  4. The photoelectric emission is possible only if the incident light is in the form of packets of energy, each having a definite value, more than the work function of the metal.
  5. The intensity of light is depends upon the no. of photons present in it. The rest mass of photon is zero.
  6. Photoelectric effect is a device which converts light energy into electrical energy. It is also called an electric eye.
  7. De-Broglie hypothesis: according to him a moving material particle sometimes acts as a wave and sometimes as a particle or a wave is associated with moving material particles which controls the particle in every respect. The wave associated with whose wavelength called the de-Broglie wavelength is, Lambda=h/mv
  8. De-Broglie waves cannot be electromagnetic in nature because electromagnetic waves are produced by motion of charged particles.
  9. Experimental demonstration of wave nature of electron was done by Davisson and Germer in 1927. Lambda =12.27/under root V. Electron microscope is based on the de-Broglie hypothesis.
  10. Alkali metal surfaces are most suited as photosensitive surfaces because they have very low work function.
  11. Non metals show photo electric effect with light of high frequency.
  12. Photo-electric effect and Compton effects both illustrate the particle nature of light.

 

Atoms and Nuclei

  1. Dalton postulated that matter is made up of atoms, which are invisible.
  2. JJ Thomson proposed the structure of the atom, which was modified by the Rutherford and later by Niels Bohr.
  3. JJ Thomson water melon model. Its limitations are that it could not explain the origin of spectral series of hydrogen and other atoms, observed experimentally and it could also not explain large angle of scattering of alpha particles from thin metal foils, as observed by Rutherford.
  4. Rutherford’s Alpha ray scattering experiment: discovery of atomic nucleus.
  5. Rutherford postulated that the entire positive charge of the atom must be concentrated in a tiny central core of the atom. This tiny central core of each atom was called atomic nucleus.
  6. The scattering of alpha particle is due to elastic collision between nucleus and alpha particles.
  7. Balmer series was discovered in the visible region of the hydrogen spectrum. He gave an empirical formula= 1/lamba= R*(1/2^2-1/n^2). R is Rydberg constant = 1.097 * 10^7 per meter.
  8. Lyman series was discovered in Ultraviolet region of hydrogen spectrum. Paschen series was discovered in Infrared region of hydrogen spectrum. Brackett series was discovered in infrared region of hydrogen spectrum. Pfund series is also discovered in infrared region but very far from Pashen and Brackett.
  9. Limitations of Rutherford model are: as the revolving electron loses energy continuously, it must spiral inwards and eventually fall into the nucleus and atoms should emit continuous spectrum, but what we observe is only a line spectrum.
  10. Bohr postulates:
    1. Every atom consists of a central core called nucleus, in which entire positive charge and almost entire mass of the atom are concentrated. A suitable number of electrons (having as much negative charge as the positive charge on the nucleus) resolve around the nucleus in circular orbits. The centripetal force required revolution is provided by the electrostatic force of attraction between the electron and the nucleus.
    2. Stable orbits: according to the Bohr, electron can revolve only in certain discrete non radiating orbits, called stationary orbits, for which total angular momentum of the revolving electron is an integral multiple of h/2pie, where h is plank constant i.e. m*v*r=n*h/2pie. N is principal quantum number.
    3. The emission/absorption of energy occurs only when an electron jumps from one of its specified radiating orbit to another. The difference is the total energy of electron in the two permitted orbits absorbed when the electron jumps from an inner to an outer orbit, and when electron jumps from outer to the inner orbit. h*v= E1-E2
    4. Total energy of electron in Bohr’s stationary orbit is E= -13.6eV/n^2
  11. In Bohr’s theory of hydrogen atom, electron revolving in any stationary orbit is bound to the nucleus and cannot leave the nucleus on its own. This is the implication of the fact that the potential energy is negative and is greater in magnitude than the kinetic energy.
  12. Angular momentum has the same dimension as that of Plank’s constant.
  13. The Energy possessed by an electron for n= infinity is zero.
  14. 10^6 is the order of the velocity of electron in a hydrogen atom in ground state.
  15. 6eV is the ionization potential of hydrogen atom.
  16. One atomic mass unit is defined as 1/12th of the mass of an atom of 6C12 isotope. 1 a.m.u= 1.66*10^-27 kg.
  17. eV is the unit of energy.
  18. 1 a.m.u = 931MeV.
  19. The study of radioactive disintegration indicates the emission of alpha, beta and gamma particles rays to be of nuclear origin.
  20. Volume of a nucleus is directly proportional to its mass number. Therefore, atomic nuclei of different elements have different sizes. Density of nuclear matter is same for all nuclei.
  21. Binding energy of a nucleus is the energy with which nucleons are bound in the nucleus. It is measured by the work required to be done to separate the nucleons an infinite distance apart from the nucleus, so that they may not interact with each other.
  22. The force that determines the motion of atomic electrons is the coulomb force of attraction between electrons and the nucleus. Nuclear forces are the strong forces of attraction which hold together the nucleons (protons and neutrons) in the tiny nucleus of an atom, inspite of strong electrostatic forces of repulsion between protons.
  23. Nuclear forces are independent of charge. Short range; non central; non conservative force; not obeying inverse square law.
  24. The stability of a nucleus is determined by the value of its binding energy per nucleon. Higher the Binding energy per nucleon, more stable is the nucleus.
  25. Heavy nuclei, on the other hand, are stable only when they have more neutrons than protons.
  26. Radioactivity is, therefore, the property by virtue of which a heavy element disintegrates itself without being forced by any external agent to do so.
  27. The phenomenon of radioactivity was discovered by the Henry Becquerel in 1896. The total number of radioactive elements known at present is about 40. For instance, natural elements with atomic number greater than 82 are all radioactive. Their nuclei are thus unstable nuclei. Ex, Radium, thorium, actinium, polonium, etc.
  28. Alpha particle is equivalent to helium nucleus. Because of large mass, the penetrating power of alpha particles is very small. Because of large mass and velocity they have large ionizing power. Alpha particles produce fluorescence in certain substances. They affect photographic plate slightly. They are deflected by electric and magnetic field. They cause burns on human body and on being stopped, they produce heating effect.
  29. Beta particles are fast moving electrons. They can also produce fluorescence with certain substances. They affect the photographic plate and also get deflected by electric and magnetic field.
  30. Gamma rays are not deflected by the electric and magnetic field, showing that they do not have any charge. The rest mass of gamma photon is zero. They travel with the speed of light and they are EM waves but having wavelength smaller than that of X-rays. They can produce fluorescence. It cause nuclear reaction and affect a photographic plate more than Beta particles.
  31. Radioactivity is the spontaneous process which does not depend upon external factors. The number of atoms disintegrated per second at any instant is directly proportional to the number of radioactive atoms actually present in the sample at that instant. This is also known as radioactive decay lay.
  32. Unit of radioactivity: Curie; Becquerel and Rutherford.
  33. PET: Positron Emission Tomography. PET scans are used in examining the brain.
  34. A nuclear reaction represents the transformation of one stable nucleus into another nucleus by bombarding the former with suitable high energy particles.
  35. Neutron is most effective as a bullet in nuclear reaction because it carries no charge. It can hit the nucleus directly without being repelled and attracted by the nucleus or electrons.
  36. A photon is a quantum of energy. Its charge is zero, rest mass is zero, and spin is zero. Its energy depends only on its frequency. It has no antiparticles.
  37. A neutrino is a particle emitted during the beta decay. Its charge is zero, rest mass is zero, spin is ½(h/2pie). It has an antiparticle.
  38. Q value of a nuclear reaction is the energy released/absorbed in the nuclear reaction.
  39. Electron and proton have almost infinite life time.
  40. Nucleus cannot have a negative mass defect.
  41. Lighter elements are better moderators for a nuclear reactor than heavier elements because they slow down neutrons by elastic collision and it does not remove them from the core by absorbing them.
  42. In a natural uranium reactor, heavy water is preferred moderator to ordinary water because it has lesser absorption probability of neutrons than ordinary water.
  43. Cadmium rods are provided in reactor because they have a high cross section for neutron absorption. They are used for controlling the nuclear chain reaction responsible for producing nuclear energy.
  44. For fusion reaction, very high temperatures are required. It enables protons to have enough kinetic energy and overcome their mutual electrostatic repulsion and come closer than the range of nuclear forces of attraction.

Points

  1. Mica is good conductor of heat but bad conductor of electricity.
  2. A galvanometer gets converted into an ammeter when a wire of low resistance is connected in parallel with the coil of galvanometer.
  3. The pitch of sound depends only upon its frequency.
  4. Silver has maximum thermal conductivity.
  5. The image formed on retina of eye is real and inverted.
  6. Choke coil allows DC to pass through it but obstruct the AC.
  7. Total radiation Pyrometer is based on Stefan’s law.
  8. The splitting of a spectral line into several components by the application of the magnetic field is called Zeeman Effect. If it is based on electric field then it is called Stark Effect.
  9. The temperature at which both Celsius and Fahrenheit scales are the same is -40 degree.

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