Chapter 5 spectral lines of hydrogen atom

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Chapter 5 spectral lines of hydrogen atom

  1. 1. ChaPtER 5: Spectral Lines Of Hydrogen
  2. 2. SCOPE OF STUDY SUB TOPICS Energy Emitted and Absorbed In A transition Electronic Transition in Hydrogen Atom Lyman Series, Balmer Series and Paschen Series
  3. 3. ELECTRONIC TRANSITION IN HYDROGEN ATOM DEFINITION Molecular electronic transitions take place when valence electrons in a molecule are excited from one energy level to a higher energy level.
  4. 4. ELECTRONIC TRANSITION IN HYDROGEN ATOM Example Of Electronic Configuration in Metals scandium Sc [Ar] 3d1 4s2 titanium Ti [Ar] 3d2 4s2 vanadium V [Ar] 3d3 4s2 chromium Cr [Ar] 3d5 4s1 manganese Mn [Ar] 3d5 4s2 iron Fe [Ar] 3d6 4s2 cobalt Co [Ar] 3d7 4s2 nickel Ni [Ar] 3d8 4s2 copper Cu [Ar] 3d10 4s1 zinc Zn [Ar] 3d10 4s2
  5. 5. ELECTRONIC TRANSITION IN HYDROGEN ATOM In 1913, it was Neils Bohr who solved many of the problems at the time by proposing that the electron revolves around the nucleus of the atom with a definite fixed energy in a fixed path, without emitting or absorbing energy. The electron in the hydrogen atom exists only in certain definite energy levels. These energy levels are called Principal Quantum Levels, denoted by the Principal Quantum Number, n. Principal Quantum Level n = 1 is closest to the nucleus of the atom and of lowest energy.
  6. 6. ELECTRONIC TRANSITION IN HYDROGEN ATOM When the electron occupies the energy level of lowest energy the atom is said to be in its ground state. An atom can have only one ground state. If the electron occupies one of the higher energy levels then the atom is in an excited state. An atom has many excited states.
  7. 7. ENERGY EMITTED AND ABSORBED IN A TRANSITION When a gaseous hydrogen atom in its ground state is excited by an input of energy, its electron is 'promoted' from the lowest energy level to one of higher energy. The atom does not remain excited but re-emits energy as electromagnetic radiation. This is as a result of an electron 'falling' from a higher energy level to one of lower energy. This electron transition results in the release of a photon from the atom of an amount of energy (E = hn) equal to the difference in energy of the electronic energy levels involved in the transition.
  8. 8. ENERGY EMITTED AND ABSORBED IN A TRANSITION In a sample of gaseous hydrogen where there are many trillions of atoms all of the possible electron transitions from higher to lower energy levels will take place many times. A prism can now be used to separate the emitted electromagnetic radiation into its component frequencies (wavelengths or energies). These are then represented as spectral lines along an increasing frequency scale to form an atomic emission spectrum.
  9. 9. ENERGY EMITTED AND ABSORBED IN A TRANSITION A hydrogen atom in its Ground State. The electron occupies the lowest possible energy level which in the case of hydrogen is the Principal Quantum Level n = 1.
  10. 10. ENERGY EMITTED AND ABSORBED IN A TRANSITION The Bohr theory was a marvellous success in explaining the spectrum of the hydrogen atom. His calculated wavelengths agreed perfectly with the experimentally measured wavelengths of the spectral lines. More recent theories about the electronic structure of atoms have refined these ideas, but Bohr's 'model' is still very helpful to us. For clarity, it is normal to consider electron transitions from higher energy levels to the same Principal Quantum Level. The diagram below illustrates the formation of a series of spectral lines in the visible region of the spectrum of electromagnetic radiation for hydrogen, called the Balmer Series.
  11. 11. ENERGY EMITTED AND ABSORBED IN A TRANSITION
  12. 12. ENERGY EMITTED AND ABSORBED IN A TRANSITION The Bohr model for an electron transition in hydrogen between quantized energy levels with different quantum numbers n yields a photon by emission with quantum energy:
  13. 13. ENERGY EMITTED AND ABSORBED IN A TRANSITION This is often expressed in terms of the inverse wavelength or "wave number" as follows:
  14. 14. Hydrogen Spectrum
  15. 15. LYMAN SERIES, BALMER SERIES & PASCHEN SERIES As referred to above for hydrogen atoms, electron transitions form higher energy levels all to the n = 2 level produce a series of lines in the visible region of the electromagnetic spectrum, called the Balmer Series. The series of lines in the ultra-violet region, called the Lyman Series, are due to electron transitions from higher energy levels all to the n = 1 level, and these were discovered after Bohr predicted their existence.
  16. 16. LYMAN SERIES, BALMER SERIES & PASCHEN SERIES Within each series, the spectral lines get closer together with increasing frequency. This suggests that the electronic energy levels get closer the more distant they become from the nucleus of the atom. No two elements have the same atomic emission spectrum; the atomic emission spectrum of an element is like a fingerprint. Energy-level diagram below for the hydrogen atom, showing the transitions for the spectral lines of the Lyman, Balmer, and Paschen series. Each vertical arrow represents an atomic transition that gives rise to the photons of one spectral line (a single wavelength or frequency).
  17. 17. LYMAN SERIES, BALMER SERIES & PASCHEN SERIES
  18. 18. LYMAN SERIES, BALMER SERIES & PASCHEN SERIES
  19. 19. LYMAN SERIES, BALMER SERIES & PASCHEN SERIES The measured lines of the Balmer series of hydrogen in the nominal visible region are: Wavelength (nm) Relative Intensity Transition Color 383.5384 5 9 -> 2 Violet 388.9049 6 8 -> 2 Violet 397.0072 8 7 -> 2 Violet 410.174 15 6 -> 2 Violet 434.047 30 5 -> 2 Violet 486.133 80 4 -> 2 Bluegreen (cyan) 656.272 120 3 -> 2 Red 656.2852 180 3 -> 2 Red
  20. 20. LYMAN SERIES, BALMER SERIES & PASCHEN SERIES Example : Wavelength of a Lyman line. Use this figure to determine the wavelength of the first Lyman line, the transition from n = 2 to n = 1. In what region of the electromagnetic spectrum does this lie? Solution: The energy is the difference between the two levels, 10.2 eV. Then λ = hc/E = 1.22 x 10-7 m. This is an ultraviolet photon.
  21. 21. LYMAN SERIES, BALMER SERIES & PASCHEN SERIES Example : Wavelength of a Balmer line. Determine the wavelength of light emitted when a hydrogen atom makes a transition from the n = 6 to the n = 2 energy level according to the Bohr model. Solution: Using equation, we find λ = 4.10 x 10-7 nm (violet).
  22. 22. THE END ….. “if A is success in life, then A=x+y+z; Work = x; y = play; and z = keeping your mouth

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