Atomic Spectroscopy

• Visible spectrum:
  

  _	Red	Orange	Yellow	Green	Blue	Violet
  l nm	700	650	590	540	480	350
  n e14 Hz	4.29	4.62	3.23	5.56	6.25	8.57
  E eV	1.77	1.91	2.10	2.30	2.58	3.54

  A continuous visible spectrum consists of light with wavelength ranging from 700 to 350 nm. The color varies continuously from red to violet, but we generally identify 6 colors as red, orange, yellow, green, blue, violet. Typical wavelength, l, frequency n, and energy of photon E are given in the table here
  

  A continuous spectrum is similar to this one

• Line spectra of elements
  

  When electric charge passes through tubes of H, Hg, or Ne vapor, these tubes emit visible light. When light from these tube is analyzed by a prism, their spectra are as these shown here. These are typical line spectra, because each spectrum shows only a few lines, rather than a continuous spectrum (as those emitted by a hot solid).

• Line spectra of hydrogen
  

  Energy Level diagram of H atoms
  Continuum n = infinity ========== n = 6,7 ---------------- n = 5 ---------------- n = 4 ---------------- n = 3 ---------------- n = 2 ---------------- n = 1

  The visible spectra of H consists of red (656.3 nm) green, (486.1 nm), blue, (343.0 nm), indigo (410.1 nm), and violet (396.9 nm) lines.

  Balmer and Rydberg discovered that these lines can be represented by the formula:

  1 --- l	= R	1       1 (---   -   ---)   22       n2	(R = 10973731.534 m-1)

  R is the Rydberg constant. Later, a Lyman series was found in the UV region, and these lines can be represented by

  1 --- l	= R	1       1 (---   -   ---)   12       n2	(Lyman series).

  From these and other results, we know that the electrons in H atoms have the energy levels as shown in the diagram shown here.

• X-rays and metallic elements
  

  Whilhelm Conrad Röntgen (1845-1923), while experimenting with cathode rays, discovered X-rays.

  X-rays are emitted when fast-moving electrons are suddenly stopped by a target.

  A continuous spectrum is emitted when energies of electrons are low.

  Characteristic X-rays are generated when energies of electrons reach 20 keV or higher. The frequency of characteristic X-rays depends on the target.

• X-ray Diffractions
  

  Max von Laue (1879-1950) suggested to his students to use crystals to investigate the wave properties of X-rays.

  Using X-ray diffractions, W.H. Bragg (1862-1942) and W.L. Bragg (1890-1971) interpreted the salt crystal structure from the X-ray diffraction studies.

• Moseley's law
  

  Frequencies of Charactristic Lines of Some Elements
  Atomic No.	Element	Frequency /(1e18) Hz
  23	V	1.199
  24	Cr	1.310
  25	Mn	1.427
  26	Fe	1.550
  27	Co	1.677
  28	Ni	1.810
  29	Cu	1.947
  30	Zn	2.090
  42	Mo	4.021
  47	Ag	5.363
  79	Au	16.650

  Henry Gwyn-Jeffreys Moseley (1887-1915) used various metals as the target in his X-ray tubes, and measured the wavelengths of the most intense (characteristic) lines. He noticed that frequencies of characteristic lines increase with the increase of the atomic weights of the target metals. He plotted the square root of the frequencies against the order of the elements in the periodic table, and got a straight line. This is known as Moseley's law.

  The impact of Moseley's law: Characteristic lines not only confirm a substance as element, Moseley's law places the element in the proper location in the periodic table.

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E-mail: cchieh@uwaterloo.ca