Atomic Spectroscopy. Objectives

Transcription

1 Atomic Spectroscopy Name Objectives explain the difference between emission and absorption spectra calculate the energy of orbits in the Bohr model of hydrogen calculate E for energy transitions in the Bohr model of the hydrogen atom use the equations =c/ and E=h to carry out necessary calculations demonstrate appropriate precautions in the use of high-voltage gas lamps relate observed colors to regions of the electromagnetic spectrum Introduction Gaseous atoms and molecules may be converted to the plasma state by applying high voltages to samples in tubes. The color of these lamps is characteristic of the specific substance e.g. neon glows orange. The color we observe is a mixture of all of the wavelengths of radiation that are emitted. To observe individual colors of emission (emission spectra) a device is required that can separate this emitted light into a spectrum. One such device is a spectroscope. Another is a spectrophotometer. The spectroscope uses a prism and the colors are observed through an eyepiece. A spectrophotometer uses electronics and computer software to analyze and display the spectra. The insight that Bohr presented to the scientific community was the recognition that the spectral lines could be connected to transitions of the electron from one orbit to another in the Bohr model of the atom. This simplified model of the atom is limited in its ability to predict spectral behavior but it is worthy of study because it works very well for hydrogen and assists beginning chemistry students in making connections between physical observations and atomic theory. Consider the following diagram: Notice that light is emitted as an electron moves from a higher energy orbit to a lower energy orbit. The energy of an orbit increases as a function of the distance from the nucleus. Note: the symbols f and are both used for frequency of light.

2 Bohr developed an equation to calculate the energy levels of hydrogen: En = (equation one) Energy is in joules and n is an integer from 1 to infinity - a quantum number. Calculations Fill out this table: Table 1. Hydrogen Energy Levels n level Energy (Joules) Plot these energy levels on a vertical scale on a separate piece of paper. Notice that these energy levels are all negative numbers except the last one at n=infinity which is zero. This is because energy must be provided to the atom to move an electron which has a negative charge away from the nucleus which is positively charged. The most negative value of energy, the lowest in your diagram, is the one closest to the nucleus. Also notice that this is NOT a linear relationship. The energy levels are closer together as n increases. Your diagram should be labeled and have a shape something like this:

3 Complete this table. Table 2. Spectral wavelengths nf=1 nf=2 ni=6 ni=5 ni=4 ni=3 ni=2 E J nm 2.119E E J nm E J nm E J nm E J nm nf=3 nf=4 nf=5 nf=6 Note: the subscripts i and f signify initial and final position of the electron so an electron that begins in level 6 and moves to level 1 emits a photon with a wavelength of nm. Questions 1. What is the range in nanometers of the visible spectrum? 2. Energy is when electrons move from higher levels to lower levels and when electrons move from lower levels to higher levels. 3. Ionization means an electron is completely removed from an atom. Complete removal occurs when n=infinity. How much energy would be required to ionize one atom hydrogen? 4. How much energy in kj would be required to ionize a mole of hydrogen atoms?

4 Complete this table Table 3. Hydrogen Spectral Line Assignment wavelength (nm) assignment: nf ni region of spectrum ultraviolet ultraviolet Notice all of the spectral lines that are in the visible region. Observe the spectrum of hydrogen using the spectroscope demonstrated in the laboratory. Sketch all of the lines that you observe and describe the colors you see. Match them with the transitions in your table. Your observations:

5 Questions 1. What is the Balmer series? 2. What about other series for hydrogen - what are they called and what region of the electromagnetic spectrum would they occupy? 3. What color is the hydrogen lamp? Why do you think this color is observed? Spectra of Additional Elements Several other lamps will be available to you for observations. List the type of lamp and describe in detail what you observe through the spectroscope.

6 Questions 1. How do the spectra of various elements compare? What are some dominant lines in these spectra. Which elements have more spectral lines? 2. An Ocean Optics spectrophotometer will be demonstrated and you will be provided with some data to analyze to complete this report. Comment on the differences between collecting data with a spectroscope and with a spectrophotometer.