ATOMIC AND NUCLEAR PHYSICS

Geiger Marsden/ Rutherford/Charged particle scattering experiment: Beam of alpha particles fired at very thin gold foil; bombard gold atoms. Most of the particles pass through without deflection, some deflected through large angles.Much of the space in the atom is empty, positive charge highly concentrated at centre of the atom, hence large deflection. (Effect of gold atom’s electrons on alpha particle negligible as latter 8000 times larger)Alpha particle heading directly for nucleus will be reflected back along same path (do not have sufficient energy to collide with nucleus).At closest approach, alpha particle is momentarily stationary, KE converted into electrostatic PE. Hence . Thus radii of nuclei can be estimated by determining closest-approach distance for bombarding alpha particles. Limitations: Does not explain mechanism by which protons and neutrons stay inside the small nucleus, or the mechanism for radioactive decay.Use: To estimate radii of nuclei Know how to identify angle of deviation (treat separate paths of the particle as vectors, join at a point, take angle from original path) If gold nucleus replaced by another with larger nucleon number, no change in angle of deviation. Different nucleon numbers but isotopes of same element have same number of protons, no change in charge. Bohr’s Model of the Atom: Existence of atomic energy levels, electrons have discrete energies and travel in orbits at set distances from the nucleus. Electrons gain and lose energy by jumping from one orbit to another, absorbing or emitting EM radiation of energy (Bohr’s formula) . Atomic spectra: Provides evidence for quantization of energy in atomsEmission spectrum: Set of wavelengths of light emitted by the atoms of an element. Element heated, emits radiation that is passed through a diffraction grating with a spectrometer to obtain an emission spectrum. Spectrum consists of discrete bright lines of different colours on a dark photographic plate; different elements have unique emission spectrums.Bohr’s Theory: Atoms collide with ionized electrons of gas, excited atoms return to lower energy levels, photons with energy corresponding to differences of the energy levels are emitted. Hence coloured emission line spectra obtained. Absorption spectrum: Set of wavelengths of light absorbed by the atoms of an element. Spectrum consists of dark lines (absorption lines) against a bright coloured photographic plate.The dark lines are the wavelengths of light absorbed by atoms (in the gas around the sun, for absorption spectrum of sun), absorption takes place because incoming photon has right amount of energy to cause electron in gas atom to excite to a higher energy level.How it is observed experimentally: White light passed through vapour gas in a discharge tube, transmitted light passed through a diffraction grating and analyzed with a spectrometer. Results in dark lines against a photographic plate.Bohr’s theory: White light passed through cool gas in discharge tube, atoms of gas absorb photons of frequencies corresponding to energy differences of the energy levels, these frequencies are then absent from the transmitted beam. Bainbridge mass spectrometer: Can be used to find mass of nuclei.(MUST know)Existence of different nuclear masses for same element provide evidence for existence of isotopes. Atomic mass: The mass of an atom measured with reference to the carbon-12 atom.1 Unified atomic mass unit: the mass of a carbon-12 atom.Nucleons: Protons or neutronsNuclide: General term applied to a unique atom.Mass/Nucleon number: Total number of protons and neutrons in an atomAtomic/Proton number: Number of protons in the nucleus of an atomNeutron number: Number of neutrons in the nucleus of an atomIsotopes: Nuclei with the same number of protons but different number of neutronsNatural abundance: Percentage of a particular element that consists that of a particular isotope in nature What keeps the nucleus together?PROBLEM: Gravitational attraction between nucleons too small to keep nucleus together. Coulomb repulsion between protons is significant as distance between the protons is very small. Coulomb force is long-range. SOLUTION: Strong short-ranged nuclear force involving protons and neutrons keeps nucleus together (this force is not observed anywhere outside the nucleus, essentially zero if nucleons are more than 10-15m apart). The more the number of protons, the more the number of neutrons needed to keep nucleus together. Coulomb repulsion between protons Electrostatic attraction between protons and electrons Nuclear force between protons and neutrons

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