Chapter 4 - Atomic Structure PDF

Chapter 4: Atomic Structure Book: Modern Physics by Beiser (Chapters 4, 5 and 6) Modern Physics by Kennethe S Krane Modern Physics by Richtmyer Evaluation: NO Assignment Mid-Semester : 40% End-Semester : 60% Atomic Structure The Nuclear Atom An atom is largely empty space Ele...

Chapter 4: Atomic Structure Book: Modern Physics by Beiser (Chapters 4, 5 and 6) Modern Physics by Kennethe S Krane Modern Physics by Richtmyer Evaluation: NO Assignment Mid-Semester : 40% End-Semester : 60% Atomic Structure The Nuclear Atom An atom is largely empty space Electron Orbits The planetary model of the atom and why it fails Atomic Spectra Each element has a characteristic line spectrum The Bohr Atom Electron waves in the atom Energy Levels and Spectra A photon is emitted when an electron jumps from one energy level to a lower level Correspondence Principle The greater the quantum number, the closer quantum physics approaches classical physics Nuclear Motion The nuclear mass affects the wavelengths of spectral lines Atomic Excitation How atoms absorb and emit energy water fire earth air 1. All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. 2. Atoms of any one element are different from those of any other element. 3. Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4. In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.  Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912 Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials. water fire earth air  Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron  Discovery of electron: JJ Thomson (1897) The experiment Cathode rays are negatively charged particles If ΔV (the voltage difference between the two deflection plates) = 0 there is no deflection, but when ΔV is >> 0, there is Δx deflection towards the positive plate. 𝑞(−) ∆𝑥 − = 𝑚(−) This negative particle from the cathode ray tubes was later 𝑞(+) named the electron and its mass was later determined to be ∆𝑥 + = 𝑚(+) very small (m = 9.11 x 10-31 kg). ∆𝑥 − 𝑞(−) 𝑞(+) 𝑚(+) ∆𝑥 + = / 𝑚(−) 𝑚(+) = 𝑚(−) Atoms are NOT indivisible! m(-) VA → no electron can reach the collector, so no current would be measured. If Vr < VA → if the tube is highly evacuated, most of the electrons would reach the collector and have energy |e|(VA-Vr)  Accelerating Grid Collector Hg e- Hg Electrometer VA Vr 6V Filament Supply 1.5 V 0-40 V Retarding Accelerating voltage voltage If the tube contains some gas, the electrons can loose energy via collisions with the gas atoms. - Such collisions are inelastic, i.e. electrons lose energy, which is transferred to internal energy of atoms in the gas. Thus, even in the case when Vr < VA , it is possible that the electrons would not be able to reach collector, and won’t contribute to the current.  Franck and Hertz observed the collector current as a function of VA (>Vr) when tube was filled with various gases (result for mercury gas is shown here) At first, the current increased as was expected for a typical vacuum tubes, but as ~4.9V current suddenly dropped. Then, the increase resumed until 9.8 V, and so on.  The current drops because fewer electrons reach the collector. This occurs only if the electrons undergo elastic collisions. Thus, when VA=4.9 X n Volts (n=1, 2, 3, ….) the electrons undergo inelastic collisions with Hg atoms. In inelastic collision kinetic energy of electrons is transferred to the internal energy of Hg atoms- Hg atoms absorb the energy of electrons. Why do we see the drop only at specific voltages? →If distribution of energy levels of Hg atoms is continuous, then KE should be transferred to Hg atoms regardless of the energy of electrons. →However, if we assume that Hg energy levels are discrete, then only when electrons reach certain energy they undergo inelastic collision with Hg atoms. → Thus energy spectrum of Hg atom is such that an electron energy level lies ~4.9 eV above the ground state. Why did Franck and Hertz not observe dips in the current at other voltage? →Their experiment was not sufficiently sensitive. →Since as soon as electrons gain energy of 4.9eV they transfer it to the Hg atoms, only small fraction of the electrons could have higher energies (e.g 6.65eV), making it difficult to observe current dips associated with other (higher) voltages. Chapter 4 Ends